There was variation in the activity of fumarase, aconitase, isocitric dehydrogenase and malic dehydrogenase in spermatozoa from different strains of fowl. Homozygosity for rose comb was associated with a significant effect on the level of fumarase, aconitase and isocitric dehydrogenase activity in spermatozoa. Semen from males with this genetic characteristic produced poor fertility; the duration of the fertile period in the hen and the overall fertility were reduced. There was no significant correlation between the activity of any of the four enzymes in spermatozoa. Fumarase activity was significantly correlated with fertility. Duration of fertility in hens and the general level of fertility after single inseminations were correlated. It was concluded that the low level of fumarase activity in spermatozoa from homozygous rose comb males may be at least part of the reason for the low fertility produced by their semen.