Hatchability differences between breeds could be due to a number of reasons; however, egg fertility from a natural mating flock can be measured by assessing the number of sperm laid down in the outer perivitelline membrane of oviposited eggs. Although eggs have to be sacrificed, a minimum number of sperms are associated with a fertile egg and can give an indication of flock fertility. Differences in hatchability between two laying hen strains were reported by a breeder and egg fertility was investigated as a possible cause. No differences between number of sperms found in the perivitelline membrane were observed between strains, and the differences in hatchability could not be attributed to fertility. By eliminating fertility, other areas of investigation are opened to determine the causes of differences in hatchability.