Experiments were conducted at Coral Bay on the Ningaloo Reef, Western Aus-tralia (Lat. 23090S; Lon. 113460E). Acropora millepora is a simultaneous her-maphrodite, releasing eggs and sperm together, but with little or no evidence of subsequent self-fertilisation. This reef-building species is common and frequently abundant in tropical waters of the Indian and Paci®c Oceans. Six live colonies of Acropora millepora (approx. 40cm in diameter) were collected from fringing reefs on 19 March, 1999. Acropora millepora colonies were isolated in individual 50 l plastic tubs of seawater prior to sunset and synchronous spawning occurred between 21:00 and 21:30 h. The released gametes, in the form of buoyant egg-sperm bundles 1±1.5 mm in diameter, were collected in individual 250 ml jars using gentle suction from the water surface of each tub.