AbstractFifty‐seven epiphytic species were found to occur on the two sea‐grass hostsZosteraandPosidoniain Botany Bay and Jervis Bay, N.S.W. Among the algal epiphytes onPosidonia australisseveral indicator species were identified and it was found that their relative frequency of occurrence can be used to differentiate sites within the two bays. These indicator species can be used also to classify a collection from an unknown site within the region, and their relative occurrences may provide a useful index for early detection of changes in the bay environme