Established models of fertilization kinetics in free-spawning marine invertebrates predict that fertilisation success is dependent upon sperm swimming velocity. Despite the prevalence of these models, there are very few published tests of this assumption. To test this, the effects of sperm swimming characteristics on fertilisation success were studied in Galeolaria caespitosa (Polychaeta, Serpulidae). Both sperm activity (% motility) and sperm swimming velocities were highly variable within this species. Sperm were motile for up to 6-7 hours after activation, however mean motility, swimming velocity and fertilisation success decreased after 4 hours. Eggs of G. caespitosa were fertilizable up to 10 hours after spawning, however the number of embryos resulting from fertilisations by fresh sperm also decreased after 4 hours. Sperm motility and velocity was not affected by water-soluble egg extracts. When gamete concentration, contact time, and age were held constant, fertilization success in Galeolaria caespitosa remained highly variable. Some of this variability was attributable to observed variability in sperm motility and velocity. Fertilization success was positively related to sperm velocity (P = 0.014), but there was no correlation between percent sperm motility and fertilisation success (P = 0.85). Cross-fertilisation experiments indicated that individual egg fertilisability and male-female compatibility also play an important role in determining the fertilisation success and thus may obscure any effects of sperm motility.