Seventeen species of hydroids were found associated with the seagrass Amphibolis at two sites in southern Australia. Thirteen species are endemic to Australia and five species are host-specific to seagrasses. Dominant species in the leaf clusters were: Plumularia australis, Plumularia setaceoides var. crateriformis, Orthopyxis tincta, Silicularia undulata, Lineolaria spinulosa and Aglaophenia plumosa. These contributed 86 % of the hydroids colonies found. No athecate hydroids were found. Most species were present throughout the year but abundance was related to seasonal changes in water temperature with most species being most abundant between 13-20 "C. Selection between the habitats of A. antarctica and A. griffithii was influenced by size, morphology, and arrangement of leaves in the leaf-clusters. Leaf turnover and shedding of leaves in storms were important determinants of the life-span of hydroid colonies. Younger, innermost leaves were first colonised by L. spinulosa, and then by P. australis and O. tirzcta as the leaves aged. No asexual propagation of colonies was found, the obligate species showed opportunistic characters of rapid growth, early sexual maturity and prolific ova production to overcome the problems posed by a short-lived substrate.