The reproductive biology of Goniastrea aspera at Magnetic Island was compared with that of a very similar sympatric species, G. favulus at Heron Island. The development of gametes was similar in both species. Other reproductive differences between the two species were found in egg size and the mode of spawning. The eggs of G. aspera are smaller and more numerous than those of G. favulus, G. aspera expelled buoyant packets of eggs and sperm; G. favulus had sticky sinking eggs which were released separately from the sperm. The spatial pattern of the two species was examined on the reef flat at Magnetic Island to determine whether the spawning behaviour and egg buoyancy might have an effect on egg retention and the distribution of adult colonies. The results of this comparison failed to detect any difference in the degree of aggregation of the two species.