The directed-inquiry approach to learning science process skills and scientific problem solving [DIAL(SPS)2] was developed to help high-school students gain the critical thinking skills required to solve problems in the biology lab. This curriculum integrated several learning strategies into a single approach: advance organizers, the learning cycle, concept maps, Vee diagrams, a focusing strategy, and writing. Two general questions were addressed. First, was the DIAL(SPS)2 treatment more effective than a conventional treatment? Second, was there evidence of an aptitude-treatment interaction? Four high-school biology classes taught by this investigator were used to test the DIAL(SPS)2 curriculum. Scheduling of students involved ability grouping. To test the curriculum in the most rigorous way, the experimental group consisted of average ability students and the comparison group consisted of above-average students. Both the groups were pretested in August and posttested in May. In the intervening time, the experimental group received the DIAL(SPS)2 treatment while the comparison group received a more traditional approach. Analysis of covariance revealed that the DIAL(SPS)2 curriculum had no significant effect on the learning of science process skills or on cognitive development. Aptitude-treatment interaction analyses revealed an interaction of DIAL(SPS)2 treatment and cognitive development.