diff --git a/lib/LessParser.js b/lib/LessParser.js
index 098cc4e..e0707db 100644
--- a/lib/LessParser.js
+++ b/lib/LessParser.js
@@ -177,6 +177,23 @@ module.exports = class LessParser extends Parser {
       }
     }
 
+    // Test for the mixin definition pattern
+    // .mixin-name() {
+    // .mixin-name(@param1; @param2) {
+    // .mixin-name(@param1; @param2) when (@condition) {
+    // .mixin-name(@param1; @param2) when (@condition) and (@condition) {
+    if (tokens[0][0] === 'word' && isMixinToken(tokens[0])) {
+      const bracketsIndex = tokens.findIndex((t) => t[0] === 'brackets');
+      const firstParenIndex = tokens.findIndex((t) => t[0] === '(');
+      const spaceIndex = tokens.findIndex((t) => t[0] === 'space');
+
+      if ((spaceIndex === 1 && (firstParenIndex === 2 || bracketsIndex === 2)) ||
+        firstParenIndex === 1 || bracketsIndex === 1) {
+        this.mixin(tokens);
+        return;
+      }
+    }
+
     super.rule(tokens);
 
     // #123: add `extend` decorator to nodes
diff --git a/test/parser/interpolation.test.js b/test/parser/interpolation.test.js
index 5b9b0be..2bab4ab 100644
--- a/test/parser/interpolation.test.js
+++ b/test/parser/interpolation.test.js
@@ -24,7 +24,9 @@ test('parses mixin interpolation', (t) => {
   const less = '.browser-prefix(@prop, @args) {\n @{prop}: @args;\n}';
   const root = parse(less);
 
-  t.is(root.first.selector, '.browser-prefix(@prop, @args)');
+  t.is(root.first.type, 'atrule');
+  t.is(root.first.name, 'browser-prefix');
+  t.is(root.first.params, '(@prop, @args)');
   t.is(root.first.first.prop, '@{prop}');
   t.is(root.first.first.value, '@args');
 });
@@ -77,7 +79,9 @@ test('parses escaping', (t) => {
 
   const root = parse(code);
   const { first } = root;
-  t.is(first.selector, '.m_transition (...)');
+  t.is(first.type, 'atrule');
+  t.is(first.name, 'm_transition');
+  t.is(first.params, '(...)');
   t.is(first.first.name, 'props');
   t.is(first.first.value, '~`"@{arguments}".replace(/[[]]/g, \'\')`');
   t.is(root.nodes[1].first.selector, '& ~ .stock-bar__content .stock-bar__control_pause');
diff --git a/test/parser/mixins-definition.test.js b/test/parser/mixins-definition.test.js
new file mode 100644
index 0000000..6269581
--- /dev/null
+++ b/test/parser/mixins-definition.test.js
@@ -0,0 +1,58 @@
+const test = require('ava');
+
+const { parse } = require('../../lib');
+
+test('with empty params', async (t) => {
+  const less = '.foo() { color: red; }';
+  const { first } = parse(less);
+
+  t.is(first.type, 'atrule');
+  t.is(first.name, 'foo');
+  t.is(first.params, '()');
+
+  console.log(first);
+});
+
+test('with one param', async (t) => {
+  const less = '.foo(@_color: red) { color: @_color; }';
+  const { first } = parse(less);
+
+  t.is(first.type, 'atrule');
+  t.is(first.name, 'foo');
+  t.is(first.params, '(@_color: red)');
+});
+
+test('with two params', async (t) => {
+  const less = '.foo(@_color: red; @_width: 100px) { color: @_color; width: @_width; }';
+  const { first } = parse(less);
+
+  t.is(first.type, 'atrule');
+  t.is(first.name, 'foo');
+  t.is(first.params, '(@_color: red; @_width: 100px)');
+});
+
+test('with params and when condition', async (t) => {
+  const less = '.foo(@_color: red; @_width: 100px) when (@mode = huge) { color: @_color; width: @_width; }';
+  const { first } = parse(less);
+
+  t.is(first.type, 'atrule');
+  t.is(first.name, 'foo');
+  t.is(first.params, '(@_color: red; @_width: 100px) when (@mode = huge)');
+});
+
+test('with params and 2x when condition', async (t) => {
+  const less = '.foo(@_color: red; @_width: 100px) when (@mode = huge) and (@mode = small) { color: @_color; width: @_width; }';
+  const { first } = parse(less);
+
+  t.is(first.type, 'atrule');
+  t.is(first.name, 'foo');
+  t.is(first.params, '(@_color: red; @_width: 100px) when (@mode = huge) and (@mode = small)');
+});
+
+test('avoids parsing pseudos as mixins', async (t) => {
+  const less = '.foo:is(input) { color: red; }';
+  const { first } = parse(less);
+
+  t.is(first.type, 'rule');
+  t.is(first.selector, '.foo:is(input)');
+});
diff --git a/test/parser/mixins.test.js b/test/parser/mixins.test.js
index c218ddb..5bfe809 100644
--- a/test/parser/mixins.test.js
+++ b/test/parser/mixins.test.js
@@ -9,8 +9,9 @@ test('basic', (t) => {
   const root = parse(less);
   const { first } = root;
 
-  t.is(first.type, 'rule');
-  t.is(first.selector, selector);
+  t.is(first.type, 'atrule');
+  t.is(first.name, 'foo');
+  t.is(first.params, params);
   t.is(first.first.prop, 'border');
   t.is(first.first.value, 'baz');
   t.is(nodeToString(root), less);
@@ -128,8 +129,14 @@ test('guarded namespaces', (t) => {
   const root = parse(less);
   const { first } = root;
 
-  t.is(first.first.selector, '.mixin()');
-  t.is(first.next().first.selector, '.mixin() when (@mode=huge)');
+  t.is(first.first.type, 'atrule');
+  t.is(first.first.name, 'mixin');
+  t.is(first.first.params, '()');
+
+  const next = first.next();
+  t.is(next.first.type, 'atrule');
+  t.is(next.first.name, 'mixin');
+  t.is(next.first.params, '() when (@mode=huge)');
   t.is(nodeToString(root), less);
 });