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 byexplicit instruction, on the acquisition of implicit second language (L2) knowledge. This studyinvestigates the interaction between the presence or absence of explicit instruction and thevariables complexity and morphology/syntax in the acquisition of four L2 structures. Twogroups of 27 university students, differing in the exposure to explicit instruction, followed acomputer-controlled self-study course in an artificial language. Results from computer-controlledposttests confirm the general hypothesis that explicit instruction facilitates the acquisition of L2grammar. However, no evidence could be reported for the hypotheses predicting a differentialeffect of explicit instruction depending on the variables complexity andmorphology/syntax.