In inbred male mice sampled at a standard time of year, increased age was accompanied by a marked fall in the proportion of spermatozoa with normal acrosomal caps, thus indicating a concomitant fall in fertility. Age trends were noted in the projected area of the midpiece and perhaps also in the breadth of the spermatozoan head. There was no significant age trend in the projected area of the spermatozoan head or the length of the midpiece. No difference of any kind was detected between the spermatozoa of albino and heterozygous albino males.