The epiphytic macro-algal assemblages on leaves of Posidonia sinuosa Cambridge et Kuo were examined over an annual cycle at two locations near Perth, Western Australia. The two locations were considered to be representative of inshore protected (Owen Anchorage) and offshore semi-exposed (Success Bank) areas.Strong seasonal trends in diversity and structure of the assemblage occurred, but seasonal peaks in diversity and structure were different at the two locations. The epiphytic macroalgal assemblages at Owen Anchorage peaked in diversity and relative abundance of component species during summer and autumn (December 1992 to June 1993) whereas 8 km away at Success Bank the peak was in the autumn (February and April 1993). Multivariate clustering and non-metric multidimensional scaling indicated that species composition of the assemblages were different between the 2 locations. Owen Anchorage assemblages were represented by filamentous Rhodophyta from the Rhodomelaceae, filamentous Phaeophyta and Cyanophyta. Success Bank assemblages were represented by articulated and crustose coralline algae (Corallinales) and filamentous Rhodophyta from the Ceramiaceae. Seasonal differences between the 2 locations were related to differences in seasonality in leaf elongation rates, measured as change in leaf length between sampling periods, although grazing pressure during spring was also implicated. Differences in assemblage composition between the 2 locations was discussed in relation to differences in wave exposure and in recruitment regimes at the two locations: Owen Anchorage having more sources of recruiting propagules (reproducing epiphytes) than Success Bank.