The present article reports on the findings of a study that explored the effect of explicit strategy based instruction on achievement test scores in mixed language ability groups within an ESP course in a higher education setting. The research results indicate that language learner strategy instruction, which focused on cognitive, metacognitive and memory strategies, did not have any effect on achievement test scores among students with a higher level of general language competence. The results also indicate that membership in the experimental group was not a positive predictor of scores on vocabulary tasks of the achievement test among students with lower levels of pre-existing language ability. The results, discussed with respect to the context in which strategy based instruction was conducted, bring into question the justification for explicit strategy based instruction in mixed language ability groups, emphasize the importance of metacognitive awareness, and suggest that when insufficient time is available for integrated strategy instruction, a separate and independent module on learner strategies, focusing on different strategies for students at different levels of language competence, or implicit language learner strategy instruction seem to be more appropriate.