A study explored a potential aptitude-by-treatment interaction associated with the keyword method of vocabulary acquisition. This method is a two-stage mnemonic process whereby an unfamiliar term is first transformed into a familiar concrete stimulus and then a thematic relationship is created between the transformed stimulus and the information associated with the original term. Subjects were 144 fifth grade students with either high or low levels of vocabulary knowledge who were randomly assigned either to one of three keyword instructional conditions that varied in the degree of structure provided or to a no-strategy control condition. All subjects were asked to learn 16 new vocabulary words. Results showed that all three variations of the keyword method facilitated students' vocabulary learning. However, aptitude-by-treatment interactions involving vocabulary knowledge materialized in such a way that the degree of keyword structure made far less difference for the high knowledge students than it did for the low knowledge subjects. In particular, when the students had to execute the dual components of the keyword method entirely on their own, low vocabulary knowledge students experienced considerably more difficulty than did the high knowledge students. (FL)