A reactimeter has been developed to measure the neutron reactivity of the Virginia Polytechnic Institute and State University nuclear research reactor. The reactimeter will be employed in monitoring reactivity changes of samples entering and leaving the reactor. The reactimeter is comprised of a compensated ion chamber that measures the neutron flux of the reactor and a microcomputer that performs the reactivity calculations. The calculations are based on the six group, point reactor kinetics equations. To simplify the algorithm programming into the microcomputer, the prompt jump approximation is used. The entire reactimeter program can be stored in 2 K of memory, but it requires a separate program of elementary mathematical subroutines. This second program performs all the mathematical operations and requires 1.25 K of memory. / Master of Science