The lions of Lake Manyara National Park were observed for 1 1/4 years. About 0.4 lions/km2 inhabited the park, a density nearly double that found in Nairobi National Park and Ngorongoro Crater. One group of lions occupied the southern half of the park; in the northern half, the range of two males included two distinct groups of lionesses each of which confined itself to its own area. The composition and changes in composition of each group are discussed. Buffalo were observed to kill a lion cub on three occasions, and elephant were seen to do so once. The lions' most important prey was buffalo, which comprised 62% of the kills, followed by zebra (18%) and impala (12%). Eighty-one per cent. of the buffalo killed were males. Lions were seen in trees on 2/3 of the occasions on which they were encountered. Possible reasons for tree-climbing are given.