This study examined the effectiveness of explicit instruction on second language (L2) learners' implicit and explicit knowledge of English. Explicit instruction on the generic and non-generic use of English articles was delivered by CALL activities. Four tasks assessed acquisition: elicited imitation, oral production, grammaticality judgement, and metalinguistic knowledge tasks. A pretest and two posttests were conducted immediately and six weeks after the treatment. Durable effects for explicit instruction were found on measures of implicit knowledge and on ungrammatically supplied items on measures of explicit knowledge. This study's findings contribute towards our understanding of the efficacy of explicit instruction on implicit and explicit knowledge at relatively advanced stages of L2 acquisition. (Contains 10 tables and 2 notes.)