ABSTRACT: In this study, we aim to find out the effects of computer-aided instruction (CAI) on the students' achievement in biology and their attitudes toward the use of computers in biology courses as compared to the traditional methods. In order to find out these effects, an experimental study has been carried out in Kuleli Military High School in Istanbul. The subjects of the study were 51 students of the first graders from the two sections of the high school. In the control group, first the subject "enzymes" was taught and then the students studied the subject through a traditional method using worksheets. In the experimental group, CAI was implemented. These students were given the chance of studying the same subject through a selected courseware (Vitamin Biology, a tutor) in the computer laboratory. An achievement test has been designed to measure the success of the students and an attitude scale has been used to display their attitudes toward computer use. The test and the scale have been applied to both groups before and after the instruction. The differences between the means of the pre and post tests have been compared via t-test. No significant difference has been found out. It can be concluded that CAI does not have much effect on the students' achievement in biology. The same procedure has been followed in interpreting the results of the attitude scale and no significant difference has been observed.