Gametogenesis and reproductive behaviour of Acanthaster planci were observed at Davies Reef between September 1990 and February 1991. Gonad development in females and males of A. planci peaked in early December 1991. Mean gonad volume then decreased by about 50% following a major spawning in which at least two-thirds of the population participated. Mean gonad volume recovered to near prespawning values by mid January, despite a further minor spawning that was observed in the population. Further spawnings are likely to have occurred in mid- to late January. Fertilization rates measured during natural spawnings of A. planci were highest (83%) in samples taken at the peak of a major spawning event and much lower (23%) in samples taken at the end of the same spawning event. Trials conducted with starfish artificially induced to spawn indicated that potential fertilization rates decreased relatively slowly as distances between female and male animals increased. Fertilization rates were 47% for animals separated by 32 m. For animals separated by more than 60 m, fertilization rates were 23%. The distances at which these fertilization rates were achieved were two orders of magnitude greater than those recorded for other marine organisms, most likely because of the large volumes of gametes that A. planci can release.