We describe the spawning behaviour and some aspects of spawning periodicity in a diverse group of marine invertebrates, principally echinoderms, but including sponges, anthozoans, molluscs, and polychaetes. Our observations were made both opportunistically and on a systematic basis between 1978 and 1992 on the central and northern Great Barrier Reef. Spawning was predictable in some of the species observed, for exampleBohadschia argus, Euapta godeffroyi, andStichopus variegatus(Holothuroidea), which exhibited regular lunar and diel periodicity. Others, such asHolothuria coluber, Actinopyga lecanora, andBohadschia graffei(Holothuroidea),Acanthaster planei(Asteroidea),Hyotissa hyotisandArcaspp. (Bivalvia) exhibited no clear lunar or diel periodicity in spawning behaviour. Mass heterospecific spawnings which involved several species, often from different phyla, were commonly observed. The species participating were usually those with unpredictable spawning patterns and while the species involved were diverse there were also occasions when spawning involved species from the same genera. Fertilization rates were measuredin situfor the predictable spawnerBohadschia argusand were found to vary between 0–96% depending on the circumstances of the spawnings. Fertilization rates for the unpredictable spawners showed similar variability;Holothuria coluberandActinopyga lecanoraranged from 9–83%.