This study examined the differential effectiveness of traditional and discovery methods of instruction for the teaching of science concepts, understandings about science, and scientific attitudes, to learners at the concrete and formal level of cognitive development. The dependent variables were achievement, understanding science, and scientific attitude; assessed through the use of the ACS Achievement Test (high school chemistry, Form 1979), the Test on Understanding Science (Form W), and the Test on Scientific Attitude, respectively. Mode of instruction and cognitive development were the independent variables. Subjects were 120 Form IV (11th grade) males enrolled in chemistry classes in Lusaka, Zambia. Sixty of these were concrete reasoners (mean age = 18.23) randomly selected from one of the two schools. The remaining 60 subjects were formal reasoners (mean age 18.06) randomly selected from a second boys' school. Each of these two groups was randomly split into two subgroups with 30 subjects. Traditional and discovery approaches were randomly assigned to the two subgroups of concrete reasoners and to the two subgroups of formal reasoners. Prior to instruction, the subjects were pretested using the ACS Achievement Test, the Test on Understanding Science, and the Test on Scientific Attitude. Subjects received instruction covering eight chemistry topics during approximately 10 weeks. Posttests followed using the same standard tests. Two-way analysis of covariance, with pretest scores serving as covariates was used and 0.05 level of significant was accepted. Tukey WSD technique was used as a follow-up test where applicable. It was found that (1) for the formal reasoners, the discovery group earned significantly higher understanding science scores than the traditional group. For the concrete reasoners mode of instruction did not make a difference; (2) overall, formal reasoners earned significantly higher achievement scores than concrete reasoners; (3) in general, subjects taught by the discovery approach earned significantly higher scientific attitude scores than those taught by the traditional approach. The traditional group outperformed the discovery group in achievement scores. It was concluded that the traditional approach might be an efficient instructional mode for the teaching of scientific facts and principles to high school students, while the discovery approach seemed to be more suitable for teaching scientific attitudes and for promoting understanding about science and scientists among formal operational learners.