When hens were inseminated with an equal number of spermatozoa from one of 3 Leghorn (L) and one of 3 Columbian (C) cocks in the 9 possible L and C combination, a hierarchy of fertility was established based on the proportion of chicks sired by each cock. A similar hierarchy was established for 3 Duroc (D) and 3 Yorkshire (Y) boars by mating gilts in rapid succession to one D and one Y boar. A second hierarchy of fertility was established by inseminating hens with 40 x 10(6) spermatozoa from only one cock or by mating gilts to only one of the boars. The hierarchies for the cocks and boars were essentially the same for each method. Minor discrepancies were observed for males which appeared to be nearly equally fertile when used alone or in combination with another male. After homospermic insemination, the hierarchy of cocks was identical whether the ranking was based on the percentage of eggs fertilized or on hatchability of fertilized eggs. Similarly, the boars ranked highest in fertility by double mating had higher conception rates, higher embryonic survival rates and larger litter sizes when used alone. Heterospermic insemination and double mating appear to be more efficient and sensitive than methods of estimating fertility which depend on homospermic insemination of high or low numbers of spermatozoa, single matings or the examination of various characteristics of semen. The method of heterospermic insemination or double mating offers a simple and effective method of assessing the relative fertility of males.