This paper reports the results of a study on the effects of explicit instruction, which examined the L2 development of the German case system. The study investigated to what extent the effects of instruction, if any, depend on (i) the type of linguistic performance (knowledge vs. use) and (ii) the developmental readiness of the learners. The pretest-posttest study involved 18 first-year university students of German (L1 Dutch), who were administered a grammatical judgment task (which examined their knowledge of the case rule) and an oral production task (which examined their case use). The findings uncover differences between pretest and posttests, which shows that explicit instruction improves linguistic performance in general. Further analysis reveals that the effect does not depend on ‘type of performance’, but that it does depend on ‘developmental readiness’.