This study examines how two types of word knowledge, word associations and grammatical suffix knowledge, change over time both receptively and productively. Ninety-five secondary and postsecondary Japanese students were tested on three word associations and inflectional and derivational suffixes for each of 20 verbs, once near the beginning of their academic year and once near the end. The results showed their average vocabulary gain was 330 words. The students showed rather poor knowledge of the allowable suffixes for the verbs, especially the derivative 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 produce only 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 and with total vocabulary size. The subjects overall had from 19 to 25 percentage points more receptive knowledge than productive knowledge.