Although it is widely believed that seawater is chemically well-buffered, CO2-induced acidification of the world's oceans threatens the viability of many species [1-3]. Research to date has focused on the responses of adult stages of calcifying taxa to gross pH changes relevant for the years 2200-2400 [3,4]. We investigated the consequences of exposure of gametes and larvae of the sea urchin Heliocidaris erythrogramma to CO2-induced acidification by -0.4 pH units (the upper limit of predictions for the year 2100 ), and found statistically significant reductions in sperm swimming speed and percent sperm motility. We predicted the effects of these changes using an established model , and tested fertilization success experimentally in assays using the same gametes and pH treatments. Observed reductions in fertilization success corresponded closely to model predictions (24% reduction). If general, these findings have important implications for the reproductive and population viability of broadcast spawning marine species in the future acidified ocean.