Few detailed studies have been published on genetic variation in seagrasses except those on the monoecious Zostera marina L. or the hermaphrodite Posidonia australis Hook. f. This paper presents allozyme, RFLP and reproductive biology data on Amphibolis antarctica (Labill.) Sender & Aschers, one of the 75 per cent of all seagrass species which are dioecious. Collections were made from approximately one-third of the species range in Western Australia. Its only congener, A. griffithii (J. M. Black) den Hartog, was collected from one site to provide a comparison. Flowering was observed in 25 per cent of the shoots surveyed and the average sex ratio was 3.8: 1 (F:M) which it has been suggested indicates sexual reproduction. No genetic variation was found within or between populations at 14 allozyme loci. 18S RFLPs and M13 DNA fingerprinting gave few satisfactory results but also did not exhibit any variability. Allozyme variation was observed between A. antarctica and A. griffithii, the only congeneric species. The lack of allozyme and DNA variation within A. antarctica indicates a potentially low level of outbreeding, a highly clonal reproductive system or a very efficient genetic system in A. antarctica. The hypothesis that the dioecious reproductive system evolved in seagrasses to maximize outbreeding and genetic variability, proposed by several authors, is questioned in light of these data.