This paper describes an application of Computer Assisted Instruction named 'genpbs' consisting of five modules of preliminary instruction and 19 modules of problems for students in an introductory genetics course. Each problem module consists of an average of 14 problems arranged in increasing level of difficulty. The program also contains a report module designed for use by the course instructor to obtain an on-line summary of student performance in the problem modules. The program was tested by a group of 135 students in a formal course, and student performance data on the problem modules are reported. An evaluation of the program by the students indicated a highly favorable student reaction and an overwhelming preference for the computer problems as compared to problems and solutions placed on library reserve. A slight, positive correlation was found between time spent by the students on the computer and the course numerical grade. The mean course numerical grade for students using the computer was found to be slightly higher than the corresponding mean from a previous quarter when the computer was not used, but the difference was not statistically significant. (Author)