Long-term changes of seagrass meadows (Posidonia australis Hook. f. and Zostera capricorni Aschers.) in Botany Bay, Australia, were investigated using historical aerial photography (1930-1985), field observations (1970-1987) and in situ sediment coring for fibrous remains of Posidonia. Aerial photography and field observations indicated that P. australis had undergone a steady decline in distribution in Botany Bay over the past 40 years. Between 1942 and 1984, 58% (257 ha) of Posidonia was lost from the bay's southern foreshores. By 1987, the once continuous meadow of Posidonia consisted of a number of fragmented beds. The decline occurred during a period of industrial and residential development in the catchment, which included the dredging of the bay's entrance to allow passage of large container ships. The dredging caused an increased wave height in many parts of the bay, and resulted in deterioration and erosion of seagrass beds, particularly during storm events. As an example of this process, the effect of minor storms in reducing leaf biomass of Posidonia by 70% is reported. Grazing by the sea urchin, Heliocideras erythrogramma (Valenc.), between 1980 and 1986 was responsible for smaller losses of Posidonia. The existence of fibrous remains in sediment cores offered circumstantial evidence that Posidonia had a much wider distribution in Botany Bay prior to the earliest aerial photography. A history of poor catchment management, uncontrolled effluent disposal and widespread dredging is suggested as the cause for these earlier, unquantified losses. Over the same period (1930-1987), Z. capricorni was found to have undergone cyclical fluctuations in area throughout the bay, and had colonised many sites that were previously vegetated with Posidonia. © 1990.