diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index 2178ce2e42..abf5f00059 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -25,4 +25,7 @@
     <logging>
         <log type="junit" target="tests/phpunit/build/logs/junit.xml" logIncompleteSkipped="false"/>
     </logging>
+    <php>
+        <const name="WP_RUN_CORE_TESTS" value="1" />
+    </php>
 </phpunit>
diff --git a/tests/phpunit/includes/testcase.php b/tests/phpunit/includes/testcase.php
index 42301644fb..3e4065644e 100644
--- a/tests/phpunit/includes/testcase.php
+++ b/tests/phpunit/includes/testcase.php
@@ -37,8 +37,18 @@ class WP_UnitTestCase extends PHPUnit_Framework_TestCase {
 		ini_set('display_errors', 1 );
 		$this->factory = new WP_UnitTest_Factory;
 		$this->clean_up_global_scope();
-		$this->reset_post_types();
-		$this->reset_taxonomies();
+
+		/*
+		 * When running core tests, ensure that post types and taxonomies
+		 * are reset for each test. We skip this step for non-core tests,
+		 * given the large number of plugins that register post types and
+		 * taxonomies at 'init'.
+		 */
+		if ( defined( 'WP_RUN_CORE_TESTS' ) && WP_RUN_CORE_TESTS ) {
+			$this->reset_post_types();
+			$this->reset_taxonomies();
+		}
+
 		$this->start_transaction();
 		$this->expectDeprecated();
 		add_filter( 'wp_die_handler', array( $this, 'get_wp_die_handler' ) );