Relic inter-tidal assemblages of sessile organisms which are referred to as fixed biological indicators (FBIs) are shown to be useful proxies for late Holocene climatic and environmental changes as well as direct indicators of short term fluctuations in sea-level. A detailed comparison of change in the inter-tidal assemblages at two comparable latitudinal Southern Hemisphere sites, Port Hacking south-east Australia and the Laguna-Imbituba region of southern Brazil, indicates many marked similarities in the timing and nature of changes. A similar relationship between sea level fluctuations and shifts between temperate and sub-tropical inter-tidal and sub-tidal shellfish species occurs at Rottnest Island, Western Australia. The concurrence is such that it suggests that global forcing factors are swamping regional or local influences, such as isostasy. The value of inter-tidal indicators is developed by identifying the environments associated with the present distribution of the common marker species: tubeworms, barnacles and mussels. Within the relic biostratigraphy at one particular Port Hacking site, two environmental marker species have been discovered which have important implications for late Holocene research. Firstly, the massive form of the coral worm, Idanthyrsus pennatus, now found only on the sub-tropical shoreline 400 km to the north, was present at Port Hacking for at least a 900 year period, 4300-3400 years BP. This encrustation covers an earlier formation containing barnacles and bivalves, including a second environmental marker species, the southern mussel, Brachidontes rostratus, which now occurs in colder water 240 km to the south of Port Hacking. The d ¹⁸O content of these extant specimens at Port Hacking confirm changes in ocean water temperature and associated sea-level behaviour corresponding to invasion or extinction of species. A similar pattern is repeated, although in a less marked fashion, in the Laguna-Imbituba region of southern Brazil. After 2400 years BP, which date also seems to mark a period of widespread environmental and coastal change, there is a divergence in both regions in marker species distributions and d ¹⁸O content. However, the connection between even relatively small sea level fluctuations and similar small scale climatic shifts seems to be sufficiently pronounced to provide a valuable new proxy to test against terrestrial records of environmental change.