THE evolution of egg and sperm(1,2), and more derived forms of sexual dimorphism, is thought to be driven by sperm competition and postzygotic survival; males are limited by fertilizations, females by resources(3). Evidence of sperm competition comes from internal fertilizers, or cases where sperm are deposited on eggs(4), but in free-spawners, the ancestral mating strategy (refs 1,5 but see ref. 6), females are often sperm limited(7,8). Laboratory experiments on sea urchins demonstrate that intraspecific differences in gamete attributes, such as egg size : can influence rates of fertilization. Field experiments in which gametes are released and recaptured demonstrate that the influence of gamete traits on fertilization is not overwhelmed by sea : conditions, and that variation in gamete traits can have important fitness consequences. These results suggest a new mechanism for the evolution of anisogamy and sexual dimorphism, in which sperm limitation is important, and natural selection for enhanced fertilization acts on females as well as males.