Explicit instruction can facilitate learner awareness of the surface features of a language, but does not guarantee it. Similarly, learners in an incidental learning condition are not necessarily unaware. This study investigated the development of awareness, among Japanese ESL learners, of rules of thumb for the use of zero and definite articles with place names under an explicit instruction condition, in which learners were given the rules plus examples, and an incidental instruction condition, in which learners responded to sentences containing examples. All instruction was computerized. Instruction was given in English and was followed by a twenty-question debriefing interview conducted in the learners' L1 in order to assess their awareness. The findings show that awareness could develop under either condition, but that the explicit condition was much more facilitative. The study also found a very strong relationship between awareness and improved learner performance.