After beginning his statistical research on sequential methods in 1961, Professor W.J. Ewens turned his attention to the mathematical theory of population genetics. Since 1963, he has made a large number of distinguished contributions to this field; these have been concerned with the accuracy of the diffusion equation, the problem of the self-sterile allele, the evolution of dominance, the survival of mutants, the theory of selection based on fitness, and most recently the theory of selectively neutral alleles. He is the author of two books on population genetics. His work, which has developed mathematical theory based on experimental genetic results, has earned him international recognition.