Morrumbene is a tropical estuary which opens into Inhambane Bay at 23°S on the coast of Moçambique. It is 193 km2in area and 20 km long with well-developed mangrove swamps draining into a broad lagoon. Temperature, salinity and substrate conditions are described and it is shown that water temperatures in the upper reaches vary from 18,8°C in winter to 28,2°C in summer. The vegetation is described briefly. The fauna was sampled by dredging, netting and intertidal transects along the estuary. It is very diverse. An annotated list shows the distribution of 404 macroinvertebrates and 114 species of fish. The estuary is not highly productive.