Abstract. Fertilization rates among marine benthic taxa have implicitly been assumed to be uniformly high in most analyses of life history evolution, but in situ fertilization rates during natural spawning events are rarely measured. Fertilization rates of the Caribbean gorgonians Plexaura kuna and Pseudoplexaura porosa were measured at a site in the San Bias Islands, Panama, by collecting eggs downstream of colonies during synchronous spawning events during the summer months in the years 1988-1994. Eggs collected by divers were incubated, and the proportion of eggs that developed was determined. Proportions of eggs developing suggest fertilization rates that vary from 0% to 100%. Monthly means ranged from 0% to 60.4%. Failure of gametes to develop can be attributed to sperm limitation, as eggs collected during spawning had higher fertilization rates if incubated with an excess of sperm. Plexaura kuna fertilization rates were highest during the July spawning events. Fertilization of Plexaura kuna eggs was usually lower during the first two nights of the 4-6 night spawning event. The proportion of eggs being fertilized when collected from a given place and time was highly variable, with one peak in the frequency distribution at or below 20% fertilization, and a second group of samples with greater fertilization rates. High variance in fertilization rates is evident at all levels of analysis: between replicate samples, times within nights, and among nights and months. This variance can be attributed to a combination of the effects of heterogeneity in the water