Bivalves play a vital role in estuarine ecosystems, but are vulnerable to rapid or prolonged changes in the physico-chemical environment. The St Lucia estuarine lake exhibits sub-decadal changes from wet to dry periods, resulting in fluctuating physico-chemical conditions. This paper represents a census of the diversity of bivalve molluscs in this estuary, emphasising changes associated with climate-induced hydrological states. Twenty-four bivalve species were recorded within St Lucia between 1925 and 2011. Twelve that had not been reported previously from St Lucia in the literature were encountered during the present study. These are Anadara natalensis, Anomia achaeus, Arcuatula capensis, Chambardia wahlbergi, Corbiculafluminalis, Dendostrea sandvichensis, Fulviafragilis, Mactra cuneata, Martesia striata, Meretrix meretrix, Saccostrea forskahlii and Tellina s.l. bertini. Single shells of another two previously unrecorded species, Anodontia edentula and Timoclea lavrani, were also found, although these may have been introduced dead from the ocean through tidal exchange. Meretrix meretrix, an unexpected yet abundant species, represents a new record for South African waters, but the timing and mode of its introduction into the estuary remain unresolved. Extremely large numbers ( 100 ind.[m.sup.-2]) of dead specimens of Barnea manilensis were observed in 2011 along the western shoreline of the estuarine lake, which indicates that the species was a major role-player in the settling of suspended silt within the system. Currently, this function appears to be fulfilled only by Solen cylindraceus. Apart from this species, Brachidontes virgiliae, Dosinia hepatica, Macomopsis moluccensis, Salmacoma litoralis and an undescribed Tellina sp. (reported here as T. cf. rousi) are the only bivalves that were found alive in St Lucia during the latest survey, in March 2011. KEY WORDS: Bivalvia, molluscs, South Africa, St Lucia, taxonomy, biogeography, estuarine variability, checklist.