We used egg break-out and spermatozoa trapped in the perivitelline layer of eggs to test the hypothesis that sperm supply and egg fertilization rate are high in the ostrich. Egg fertilization status was determined at break-out by the appearance of the germinal disc (GD) and then the perivitelline layer overlying the GD region was collected to count sperm (Sperm(OPVL)) under fluorescence following staining with 4',6'-diamidino-2-phenyindole (DAPI). The study was carried out on commercial ostrich farms over two laying seasons. In the first year, 229 eggs from nine randomly chosen ostrich pens comprising pairs, trios (two females, one male) and larger groups were collected for 1 week of every month of laying. Eggs contained 253 +/- 18 Sperm(OPVL)/mm(2) of the GD (mean +/- SEM; range 0-1330). Egg fertilization rate averaged 89.4 +/- 3.4% and varied from 78.6 to 98.2% between pens. Month had no effect on sperm supply or egg fertilization status. Eggs from paired birds (sex ratio 1 : 1) had less sperm in the GD than the eggs from pens with a higher sex ratio. In the second year, 150 eggs from seven pens, each containing only one male and either one, two or three females, were studied for 2 weeks at the beginning (winter), middle (spring) and end (summer) of laying. Eggs contained 364 +/- 45 Sperm(OPVL)/mm(2) of the GD (range 0-2880). Season had no effect on sperm supply or egg fertilization. The number of Sperm(OPVL) varied between pens, assumed to be due to variation between individual males. The number of Sperm(OPVL) increased as the sex ratio increased only when very high-ranking males were excluded from the analysis. Egg fertilization rate was 94.4 +/- 3.1% but varied from 64.0 to 100% between pens. Egg fertilization was not affected by season or sex ratio. Low fertilization rates were observed in two pens and appeared related to the lack of synchrony between timing of laying and sperm production in the first, and lack of mating in the second pen. We conclude that ostrich flocks generally have high rates of egg fertilization and any infertility is associated with lack of sperm supply.