Latitudinal diversity gradients are a general feature of the terrestrial realm. Fewer studies have addressed marine habitats and those concerning soft sediments have not reported such consistent trends. This study investigates global patterns of macroin-vertebrate α-diversity in estuarine tidal flats. A literature search was conducted to collect data on species diversity as well as various physical, chemical and biological factors that may prove useful in investigating the cause of trends. Regression analysis revealed a significant association between latitude and diversity expressed as Simpson's index of concentration (r2= 0.44). the index being lower (i.e. diversity higher) at low latitudes. There was no significant association between diversity and either available estuary area or annual rainfall. A significant, although weak, relationship between diversity and mean annual temperature was apparent (r2= 0. 23), together with an increase in species to family ratio in the hottest areas. This could suggest greater evolutionary speed in the tropics (due to temperature increasing mutation rates and generation times) and may provide an explanation for the trend. However, a greater amount of variation is explained by latitude alone and it is suggested that a primary cause of the latitudinal cline in estuarine diversity may be the greater effective evolutionary time available for communities in the tropics, temperate estuaries being regularly disturbed by glaciation during the last 1.8 million years.