AUTOMATIC digital electronic computers, such as the SILLIAC, have several features which are of use in simulation of genetic systems. These are the ability to form sets of pseudo-random numbers1, the ability to perform the operations of logical summation, equivalence, etc., and particularly the ability to perform a sequence of operations repeatedly at high speeds. A programme2 has been written for the SILLIAC which simulates the action of selection on a population segregating for six loci. The programme has been run for complete dominance, equal effects at all loci, and no interactions between loci. The number of progeny was set at 100, and of parents at 50. Mating was at random. The initial population was set to contain the gametic types listed in binary notation in Table 1.