The study examined the effect of concept-mapping and problem-solving teaching strategies on achievement in biology among Nigerian secondary school students. The method used for the study was a quasi-experimental pre-test treatment design. One hundred and thirteen senior secondary three (S.S. 111) students randomly selected from three mixed secondary schools located in Delta North Senatorial District of Delta State, Nigeria were used as subjects for the study. The experimental group was taught selected topics in Genetics using concept-mapping and problem-solving strategies while the control group was taught using the traditional lecture method. The result of the study showed that the experimental group performed significantly better in Genetics than the control group and that gender does not affect students achievement in biology in general. (Contains 3 tables.)