This study examined the effects of concept maps on Senior School students' achievement in ecology in Ogbomoso, Nigeria. Quasi-experimental design was employed. The population for the study was all SSII Biology students in Ogbomoso South. Four intact classes of 267 students (115 males and 152 females) were sampled. The instrument was Biology Achievement Test. The data were analyzed using "t"-test, and analysis of covariance findings of the study showed that significant difference exited in the achievement of students taught ecological concepts in biology. It was recommended that biology students should be exposed to concept maps in the learning of biology.