This study attempted to uncover, using untimed and rapid grammatical judgment tasks (GJTs)1, how constraints on verb semantics are represented in Japanese EFL learners’ explicit and implicit knowledge. Non-assertive predicates were chosen as target structures, and the participants were eighteen Japanese graduate students. In the untimed condition, the participants were allowed to take as much time as they wanted, whereas in the rapid condition they were instructed to perform as rapidly as possible. The results of a two-way ANOVA revealed that the main effect of grammaticality was statistically significant, though the main effect of task type was not. In addition, no significant interaction between the two factors was found either. This indicates that the participants were not able to correctly reject the ungrammatical sentences in both conditions. Thus, it can be concluded that the participants had neither explicit nor implicit knowledge of the rule of non-assertive predicates.