Fertilization is necessary in Ancylostoma caninum for the formation of the inner layers of the egg shell. These inner layers appear to be formed after telophase I by coalescence of secretory granules that rise to the surface of the ovum from within. Eggs that lack these layers are very fragile and easily destroyed. The ovarian end of the uterus contains many small particles. These particles seem to coalesce around the surface of the eggs after completion of telophase I. The rate of egg laying appeared to be similar for fertilized and unfertilized females. Both unfertilized and fertilized worms laid eggs in vitro and their uteri contained similar numbers of eggs. Meiosis stopped at telophase I in unfertilized eggs and was followed by degeneration of the chromosomes. The diploid chromosome number in A. caninum is 2N = 12. It is generally accepted that the most important biological function of an adult female nematode is to produce eggs, but little is known about the effect of the male on the rate of egg production. Beaver et al. (1964) used unisexual and bisexual infections of A. caninum to study blood loss and egg laying as related to mating activity of the worms. They found that sperm could be stored in the female for a period of approximately 3 weeks at which time the production of fertile eggs ceased. They further indicated that as the supply of sperm decreased the rate of egg laying appeared to decrease. The present paper provides further information concerning the rate of egg laying by A. caninum in single sex infections as compared with mixed sex infection. Ovogenesis in fertilized and unfertilized A. caninum is also