Lake Nhlange (KwaZulu, South Africa) is connected to the sea through the Kosi estuary and has a salinity between 36%0and 5%0. The benthos of the lake was sampled at approximately six-monthly intervals from July 1965 to January 1969. The substrates are mainly sandy in shallow water and also on steeply sloping profiles even in the deepest water of 30 m. Flat areas are mainly silty. In December 1965, following the closure of the estuary mouth and severe rains, the lake level rose to about 2 m above its normal level. Although the salinity changed little (5%0, to 3%0), quantities of peat-stained water and detritus were brought into the lake. A waning pattern of low oxygen conditions below about 12 m in the hot season, followed by reasonably well-oxygenated conditions in the cold season, was found. Particularly severe oxygen depletions were discovered at the mud water interface, even where well-oxygenated conditions pertained in the open water column at equivalent depths.