This study investigated the effects of computer-assisted instruction (CAI) on students' achievement in ecological concepts. Quasi-experimental design, specifically the pre-test post test non-equivalent control group design was adopted. The sample consisted of sixty-six (66) senior secondary year two (SS II) biology students, drawn from two randomly selected co-educational secondary schools in Aguata education zone of Anambra State, Nigeria. Two research questions and two hypotheses guided the study. Treatment consisted of teaching ecological concepts to the experimental group using CAI while the control group was taught using modified lecture method (MLM). Biology Achievement Test on Ecology (BATEC) was the instrument used for data collection. Mean and standard deviation were used to answer the research questions while ANCOVA was used to test the null hypothesis at 50% confidence level. Results revealed that: students' taught using CAI performed significantly better than those taught using MLM; though there was a difference in the mean achievement of male and female in both the experimental and control groups which was in favor of the males, the difference was not significant. Based on the findings of this study the use of CAI was recommended for enhancement of secondary school students' achievement in ecological concepts and other abstract concepts in science.