Fertility of domestic roosters peaked and then began a steep decline within the first year of life. The decline was concomitant with a steady reduction in total number of spermatozoa per ejaculate. We found that 1) the decline was neither related to germinal epithelium regression nor to spermatozoa abnormalities, as reported for aging males of nonseasonal breeders and for fertile seasonal breeders during the nonmating period and 2) seminiferous tubules of extremely low-fertility aging roosters contained more Sertoli cell-spermatozoa complexes than did more fertile roosters, which is in sharp contrast to what occurs in other domestic males. We further observed that Sertoli cells of low-fertility roosters had smaller diameters, cytoplasm that stained deeper, and more crowded cytoplasmic inclusions than did Sertoli cells of high-fertility roosters. We conclude that the decline of fertility in aging roosters is related to changes in Sertoli cells that impair the regular release of spermatozoa. Spermatozoa remain within the testicular tubules and thus fewer spermatozoa are available for insemination.