In second language (L2) research, the lexical unit is often defined as a base word plus inflectional and derivational forms through Level 6 of Bauer and Nation's framework (WF6). WF6 use has been justified by the assumption that once a form is known, recognition of other WF6 members requires little extra effort. A more lenient view holds that an incomplete understanding of derivational forms is permissible if words containing the most frequent derivational affixes are known. This study assessed the validity of these views for L2 listening. Participants (N = 120) provided translations of 27 base words and 43 related affixational forms when listening. When participants knew one form (either the base word or an affixed form) they also knew the other just 25.1% of the time. For target words containing the most frequent derivational affixes, this was just 26.5%. Logistic regression found that learners' overall vocabulary level, several aspects of word frequency, and base word knowledge were all significant predictors of knowing affixed forms. However, when other variables were held constant, base word knowledge was a weak predictor of affixational form knowledge. These findings support neither the strict assumption nor the more lenient view of WF6 use for L2 listening among study participants.