Theories on the role of consciousness and the enhancement of noticing (Schmidt, 1990, 1994; Sharwood Smith, 1993) predict a facilitative effect of explicit knowledge, as built up by explicit instruction, on the acquisition of implicit second language (L2) knowledge. This study investigates the interaction between the presence or absence of explicit instruction and the variables complexity and morphology/syntax in the acquisition of four L2 structures. Two groups of 27 university students, differing in the exposure to explicit instruction, followed a computer-controlled self-study course in an artificial language. Results from computer-controlled posttests confirm the general hypothesis that explicit instruction facilitates the acquisition of L2 grammar. However, no evidence could be reported for the hypotheses predicting a differential effect of explicit instruction depending on the variables complexity and morphology/syntax.