WordPress Plugin Development: How can I retrieve posts that have a specific value of an array in a custom meta key?

Anonymous basically has it, but their query is likely to return too many results. Because that data is serialized you can’t select with with an equals or IN (MySQL doesn't know it's serialized data), you have to use a LIKE. Quoting the value you're searching for makes sure it doesn't match that as a substring of something else (e.g. a is part of all, ball, aardvark). i.e. http://www.simonbattersby.com/blog/2013/03/querying-wordpress-serialized-custom-post-data/

Nick Ciske alluded to quoting the value, and he's absolutely right.  It's worth putting it here explicitly. $result = new WP_Query( array( 'post_type' => array( 'my_post_type' ), 'meta_query' => array( array( 'key' => 'my_target_meta', 'value' => '"a"', 'compare' => 'LIKE', ) ) ) ); If you wanted to get really fancy you could put in the entire serialized chunk, like this: $result = new WP_Query( array( 'post_type' => array( 'my_post_type' ), 'meta_query' => array( array( 'key' => 'my_target_meta', 'value' => 's:1:"a"', 'compare' => 'LIKE', ) ) ) ); But, maybe that's overkill.