Adult male mice had the posterior halves of their bodies exposed at 44 W/kg in a waveguide system to 2.45 GHz microwave radiation for 30 min. They were killed sequentially over 10 weeks and assessed for decreased sperm count and abnormal sperm morphology. The response in each assay was maximal 2-4 weeks after the exposure. This corresponds to microwaves having their greatest effect on spermatids and spermatocytes. Male fertility, assessed as the proportion of normal sperm per epididymis, was compared with results of an earlier study on dominant lethality. It is concluded that reduced male fertility correlates well with reduced pregnancy rate but less well with pre-implantation survival. Whilst microwaves clearly induced abnormally shaped sperm, those which achieved fertilization cannot have possessed a dominant mutation which would result in the post-implantation death of the embryo.