The presence of Posidonia australis seagrass habitat along 1000 km of the Western Australian coast provides an opportunity to study variation in epiphytic foraminiferal populations along a climatic gradient. Epiphytic foraminifera were collected in summer and winter from three nearshore shallow-water sites, which span a north-to-south gradient in climate from subtropical semi-arid to subtropical humid. Living epiphytic assemblages at each geographic site are distinct. Warmer water assemblages are characterized by Peneroplis spp. Vertebralina, Amphisorus and Marginopora, whereas Lamellodiscorbis and Rosalina spp characterize cooler water assemblages. The differences in assemblage composition partly reflect a north-to-south change in abundance of species related to climate, but also reflect local factors. Trends related to the climatic gradient include a decrease in miliolid and buliminid specimens and taxa, as well as a decrease in species with tropical affinity from north-to-south. Seasonal sampling at each geographic site also shows a decrease in miliolid and buliminid specimens and taxa, as well as a decrease in species with tropical affinity between summer and winter. This suggests that both regional and seasonal differences in epiphytic foraminiferal populations reflect a spatial vs. temporal response to change in sea temperature. Porcellaneous taxa are particularly sensitive to both regional and seasonal changes in sea temperature and appear to be good indicators of climatic setting. Using selected and critical species of foraminifera, it is possible to divide the subtropical region of Western Australia, traditionally viewed as a single biogeographic province, into at least two sub-climatic regions. Such refinement would be useful for interpreting paleoclimatic and paleoenvironmental settings of fossil assemblages.