Main hydrological conditions obtaining in Lake Nyasa are outlined. In spite of marked thermal stratification the O2 content of the hypolimnion is maintained at a considerable value due to mixing resulting from oscillations of the thermocline. The period of these oscillations is about 3-4 wks. and the amplitude about 50 m. Data are presented to support the suggestion that deep tropical lakes, which so far have only been observed in a stratified condition, may occasionally be brought to a uniform temp, during a period of several yrs. When this occurs complete mixing may result as happens annually in temperate lakes.