This study examines how two types of word knowledge, word associations and grammaticalsuffix knowledge, change over time both receptively and productively. Ninety-five secondary andpostsecondary Japanese students were tested on three word associations and inflectional andderivational suffixes for each of 20 verbs, once near the beginning of their academic year andonce near the end. The results showed their average vocabulary gain was 330 words. Thestudents showed rather poor knowledge of the allowable suffixes for the verbs, especially thederivative suffixes. Likewise, the subjects did not show very good mastery of the verbs'word associations. Even for verbs rated as known, the students as a group were able to produceonly about 50% of the word associations possible on the test as judged by native speaker norms.Word association knowledge and suffix knowledge were shown to correlate with each other andwith total vocabulary size. The subjects overall had from 19 to 25 percentage points morereceptive knowledge than productive knowledge.