Variability in gamete traits and the factors affecting fertilization success were studied in a common gregarious broadcast-spawning serpulid polychaete Galeolaria caespitosa. Variability of egg size and sperm velocity were not related to the adult size. High sperm concentrations (107-108 sperm. ml-1) were required to achieve fertilization rates of 60-80%. The gamete contact time required to achieve high fertilization rates (70-80%) in G. caespitosa did not exceed 5 minutes of gamete exposure given sufficiently high sperm concentrations. This suggests that attachment of sperm to the egg takes place very rapidly, and a short-term exposure to highly concentrated sperm is a common feature of fertilization ecology of this gregarious species. A sharp decline in fertilization rates at high sperm concentrations usually attributable to polyspermy was not observed. Sperm were motile for up to 6 hours after activation; however, swimming velocity and fertilization success decreased after 2 hours. Eggs of G. caespitosa were fertilizable up to 10 hours after spawning, but the number of embryos resulting from fertilizations by fresh sperm decreased after 2 hours. The gamete traits of G. caespitosa appear to have evolved to enable this sessile organism to reproduce under conditions of high population density and increased risk of polyspermy.