The present study sets out to measure Turkish EFL learners' receptive affix knowledge and productive derivative vocabulary knowledge. More specifically, the extent to which Turkish EFL learners recognize written form of affixes, know the meaning of affixes and determine the part of speech of the derivatives produced by means of affixes and how good such learners are at producing derivatives have been investigated. Sasao and Webb's (2017) Word Part Levels Test (WPLT) to diagnose learners' receptive affix knowledge and the Contextualized Derivative Recall Test developed by Iwaizumi and Webb (2021) to measure EFL learners' productive derivative vocabulary knowledge have been used. The findings regarding EFL learners' receptive affix knowledge reveal that EFL learners have problems with affixes with the difficulty levels of medium and hard. The further results devoted to EFL learners' productive derivative knowledge indicate that they manage to produce almost seventy-one percent of the total number of the derivatives tested. Further analysis of correct derivatives produced in terms of their grammatical class shows that nouns and verbs are the least challenging derivatives to produce for EFL learners. Last, the highest number of correctly derived forms are among the derivatives at 3K-5K word frequency bands.