A study involving 120 undergraduate students attending the University of St. Thomas in St. Paul (Minnesota) was undertaken to test the interactive effect of instructional strategy (structured versus constructed) with learner's prior domain knowledge in concept acquisition. Previous instructional design research on concept learning has focused on structured strategies (i.e., expository and practice presentations) for initial learning of concepts. The assumption has been that learners had no prior domain knowledge of the concepts to be learned. The propose of this study was to extend that research by investigating the interaction of instructional strategies with students who have prior domain knowledge. Six abstract programming concepts from the domain of structured languages were selected as the content for the learning program. Results indicate that learners with no prior domain knowledge learned concepts better with a structured strategy than with a strategy that forced learners to construct the necessary conceptual knowledge. In contrast, learners with prior domain knowledge performed better when required to construct the knowledge than did learners who experienced the structured strategy. The importance of learner prior domain knowledge is discussed as it pertains to the selection of instructional strategies. Two tables are included. (TJH)