With the introduction of computer technology into the educational system, schools have come to realize that teaching and learning can be done in a more flexible way through the application of Computer-Assisted Instruction. This method attracts greater responsiveness among students. The study investigated the influence of computer-assisted instruction and conventional instruction on academic achievements of secondary school students in Biology in national examinations in Nandi South Sub-County. Based on the study, this paper examines teachers’ and students’ attitudes towards Computer-Assisted Instruction (CAI) and conventional Instruction (CI) in secondary schools in Nandi South Sub-County. The study was based on Cognitive Theory of Multimedia Learning which states that active learning occurs when a learner engages in three cognitive processes: selection, organization and integration. The study adopted a descriptive research design approach. The target population for the study comprised all head teachers, students and teachers of Biology in all secondary schools in the Sub-County. Secondary schools were selected using simple random sampling technique while head teachers and teachers of Biology were selected using purposive sampling. The students were selected using simple random sampling technique. Questionnaires and interview schedules were the main data collection tools. Responses from all questionnaire items and interview schedule items were cross-checked to facilitate coding and processing for analysis. Responses from interview schedules were analysed descriptively. Chi-square correlation analysis was computed to investigate the relationship between attitude of teachers and students towards CAI and CI and academic achievement in Biology. The findings of the study were presented in form of cumulative frequency tables, percentages, charts and graphs. The study established that majority of students preferred that Biology be taught entirely using CAI as opposed to CI. In addition, majority of the teachers preferred CI over CAI. They believe that CAI wastes time and is more complicated. The study recommends that in order to promote the use of CAI in teaching Biology, pre-service and in-service training should be given to both teachers to help them develop instructional materials for CAI.