1. The breeding efficiency of broiler flocks was assessed by estimating the numbers of spermatozoa trapped in a 5.5 mm2 area of the perivitelline layer of laid eggs.
2. In samples of approximately 60 eggs taken from flocks on the same day, the numbers of spermatozoa ranged from 0 to several hundred.
3. Within each sample of eggs, there appeared to be two distinct populations: eggs in which perivitelline spermatozoa could not be found; and eggs containing spermatozoa for which the logarithm of the number of perivitelline spermatozoa can be approximated by a normal distribution.
4. When data from 15 flocks were considered, the overall flock fertility was linearly correlated with: the median of the number of spermatozoa per 5.5 mm of perivitelline layer (r = 0.678); the mean of positive values of the logarithm of the number of spermatozoa (r = 0.620); and the proportion of sampled eggs in which the number of perivitelline spermatozoa was greater than 0 (r = 0.714).
5. Flock age was the major factor influencing fertility and all the variables of perivitelline sperm distribution.