This study investigated the effect of Further Mathematics on students' achievement in mathematics, biology, chemistry and physics in Ogun State, Nigeria. Two Local Government Areas (LGAs) were judgmentally selected from the state. Ten secondary schools were also purposively selected from the two LGAs (Five schools from each LGA). At least twenty students (Ten Further Mathematics and Ten non-further mathematics) participated in the study in each school. Two hundred students were used for the study. The design is experimental. The further mathematics group was the experimental group with further mathematics as treatment while the control group was the non-further mathematics group. All the students selected were offering mathematics, biology, chemistry and physics. A 100-item Achievement test was administered to the students. No further mathematics test was given. Exposure to further mathematics teaching for at least two years served as treatment. There was no significant difference between further mathematics and non-further mathematics groups in achievement in each subject. However, further mathematics students had significantly better overall achievement (t = 2.012, p less than 0.05). All science students should be encouraged to offer further mathematics. (Contains 4 tables.)