diff --git a/roles/tox/README.rst b/roles/tox/README.rst
index 041d4932d..89cb3a499 100644
--- a/roles/tox/README.rst
+++ b/roles/tox/README.rst
@@ -13,9 +13,6 @@ Runs tox for a project
    ``ALL`` runs all test environments while an empty string runs
    all test environments configured with ``envlist`` in tox.
 
-   Internally this will always be expanded into a comma separated
-   list of test environments to run.
-
 .. zuul:rolevar:: tox_executable
    :default: tox
 
diff --git a/roles/tox/library/tox_install_sibling_packages.py b/roles/tox/library/tox_install_sibling_packages.py
index 4257939a9..fb6d7b95d 100644
--- a/roles/tox/library/tox_install_sibling_packages.py
+++ b/roles/tox/library/tox_install_sibling_packages.py
@@ -30,11 +30,6 @@ description:
 requirements:
   - "python >= 3.5"
 options:
-  tox_envlist:
-    description:
-      - The tox test environments to act on.
-    required: true
-    type: str
   tox_show_config:
     description:
       - Path to a file containing the output from C(tox --showconfig).
@@ -59,6 +54,7 @@ except ImportError:
 
 import pkg_resources as prAPI
 import os
+import ast
 import subprocess
 import tempfile
 import traceback
@@ -256,10 +252,28 @@ def install_siblings(envdir, projects, package_name, constraints):
     return changed
 
 
+def get_envlist(tox_config):
+    envlist = []
+    if 'tox' in tox_config.sections():
+        envlist_default = ast.literal_eval(
+            tox_config.get('tox', 'envlist_default'))
+        tox_args = ast.literal_eval(tox_config.get('tox', 'args'))
+        if 'ALL' in tox_args or not envlist_default:
+            for section in tox_config.sections():
+                if section.startswith('testenv'):
+                    envlist.append(section.split(':')[1])
+        else:
+            for testenv in envlist_default:
+                envlist.append(testenv)
+    else:
+        for section in tox_config.sections():
+            envlist.append(section.split(':')[1])
+    return envlist
+
+
 def main():
     module = AnsibleModule(
         argument_spec=dict(
-            tox_envlist=dict(required=True, type='str'),
             tox_show_config=dict(required=True, type='path'),
             tox_constraints_file=dict(type='str'),
             project_dir=dict(required=True, type='str'),
@@ -269,14 +283,12 @@ def main():
     constraints = module.params.get('tox_constraints_file')
     project_dir = module.params['project_dir']
     projects = module.params['projects']
-    tox_envlist = module.params.get('tox_envlist', '')
     tox_show_config = module.params.get('tox_show_config')
 
     tox_config = configparser.RawConfigParser()
     tox_config.read(tox_show_config)
 
-    envlist = {testenv.strip() for testenv
-               in tox_envlist.split(',')}
+    envlist = get_envlist(tox_config)
 
     if not envlist:
         module.exit_json(
@@ -306,7 +318,7 @@ def main():
 
     changed = False
     for testenv in envlist:
-        testenv_config = tox_config['testenv:{}'.format(testenv)]
+        testenv_config = tox_config["testenv:{}".format(testenv)]
         envdir = testenv_config['envdir']
         envlogdir = testenv_config['envlogdir']
         try:
diff --git a/roles/tox/tasks/main.yaml b/roles/tox/tasks/main.yaml
index 7731927f1..4d87a0aef 100644
--- a/roles/tox/tasks/main.yaml
+++ b/roles/tox/tasks/main.yaml
@@ -20,49 +20,6 @@
       UPPER_CONSTRAINTS_FILE: "{{ tox_constraints_file }}"
   when: tox_constraints_file is defined
 
-# Tox siblings cannot take 'ALL' and tox_parse_output expects
-# an envlist to be supplied so always set _tox_envlist equal to
-# the list of testenvs we're going to run
-- name: Set _tox_envlist from supplied tox_envlist
-  set_fact:
-    _tox_envlist: "{{ tox_envlist }}"
-  when:
-    - tox_envlist is defined and tox_envlist
-    - tox_envlist != "ALL"
-
-- name: Get tox default envlist
-  command: "{{ tox_executable }} -l"
-  args:
-    chdir: "{{ zuul_work_dir }}"
-  register: _tox_default_envlist
-  when: tox_envlist is not defined or not tox_envlist
-
-- name: Set tox envlist fact
-  set_fact:
-    _tox_envlist: "{{ _tox_default_envlist.stdout_lines | join(',') }}"
-  when:
-    - _tox_default_envlist is defined
-    - _tox_default_envlist.stdout_lines is defined
-
-- name: Get all tox testenvs
-  command: "{{ tox_executable }} -a"
-  args:
-    chdir: "{{ zuul_work_dir }}"
-  register: _tox_all_testenvs
-  when: tox_envlist is defined and tox_envlist == 'ALL'
-
-- name: Set tox envlist fact
-  set_fact:
-    _tox_envlist: "{{ _tox_all_testenvs.stdout_lines | join(',') }}"
-  when:
-    - _tox_all_testenvs is defined
-    - _tox_all_testenvs.stdout_lines is defined
-
-- name: Fail if tox_envlist is empty
-  fail:
-    msg: "No envlist configured in zuul or tox.ini"
-  when: _tox_envlist is not defined
-
 - name: Install tox siblings
   include_tasks: siblings.yaml
   when: tox_install_siblings
@@ -71,7 +28,9 @@
   debug:
     msg: >-
       {{ tox_executable }}
-      -e{{ _tox_envlist }}
+      {% if tox_envlist is defined and tox_envlist %}
+      -e{{ tox_envlist }}
+      {% endif %}
       {{ tox_extra_args }}
 
 - block:
@@ -81,7 +40,9 @@
       environment: "{{ tox_environment|combine(tox_constraints_env|default({})) }}"
       command: >-
         {{ tox_executable }}
-        -e{{ _tox_envlist }}
+        {% if tox_envlist is defined and tox_envlist %}
+        -e{{ tox_envlist }}
+        {% endif %}
         {{ tox_extra_args }}
       register: tox_output
 
@@ -91,7 +52,7 @@
     - name: Look for output
       tox_parse_output:
         tox_output: '{{ tox_output.stdout }}'
-        tox_envlist: '{{ _tox_envlist }}'
+        tox_envlist: '{{ tox_envlist | default("default") }}'
         workdir: '{{ zuul_work_dir }}'
       when: tox_inline_comments
       register: file_comments
diff --git a/roles/tox/tasks/siblings.yaml b/roles/tox/tasks/siblings.yaml
index 65c21bc63..8a19642c9 100644
--- a/roles/tox/tasks/siblings.yaml
+++ b/roles/tox/tasks/siblings.yaml
@@ -3,7 +3,9 @@
   command: >-
     {{ tox_executable }}
     --notest
-    -e{{ _tox_envlist }}
+    {% if tox_envlist is defined and tox_envlist %}
+    -e{{ tox_envlist }}
+    {% endif %}
   args:
     chdir: "{{ zuul_work_dir }}"
   environment: "{{ tox_environment|combine(tox_constraints_env|default({})) }}"
@@ -13,17 +15,19 @@
   tempfile:
   register: _tox_show_config_tempfile
 
-# py27, py35..py38 etc are generated on the fly if not
-# explicitly added to tox.ini, so force tox to generate
-# the config for the testenvs we're using.
 - name: Get tox envlist config
-  shell: "{{ tox_executable }} --showconfig -e {{ _tox_envlist }} > {{ _tox_show_config_tempfile.path }}"
+  shell: >-
+    {{ tox_executable }}
+    --showconfig
+    {% if tox_envlist is defined and tox_envlist %}
+    -e{{ tox_envlist }}
+    {% endif %}
+    > {{ _tox_show_config_tempfile.path }}
   args:
     chdir: "{{ zuul_work_dir }}"
 
 - name: Install any sibling python packages
   tox_install_sibling_packages:
-    tox_envlist: "{{ _tox_envlist }}"
     tox_show_config: "{{ _tox_show_config_tempfile.path }}"
     tox_constraints_file: "{{ tox_constraints_file | default(omit) }}"
     project_dir: "{{ zuul_work_dir }}"