Fertilization rates were estimated for natural spawns in the tropical wrasse, Halichoeres bivattatus. Fertilization rate averaged 88%, but varied with both sea conditions and with the addition of males (streakers) to pair spawns. As sea conditions became rougher, the mean fertilization rate for a day decreased. This effect was due to the two days with the roughest sea conditions and the lowest mean fertilization rates; there was no obvious trend when these two days were excluded from the analysis. On a given day, pair spawns, with a single male and female, had fertilization rates approximately 7% lower than spawns where 1-2 streakers joined the pair spawn. These results suggest that variation in fertilization rate must be considered as a potential selective force in shaping reproductive behavior in fishes with external fertilization of pelagic eggs.