SummaryLion population size is investigated by examining the effects of food supply and of aspects of social behaviour, on the factors which determine population size in a given area: the sizes of the lion prides there, and the sizes of the territories they occupy. Data come mainly from 7 years of records of two lion prides in the Serengeti.There is little evidence, but it is probable that territory size alters little. Pride size is best measured in terms of the number of adult females; it does not change in response to short‐term changes in food availability, and appears to be regulated.Factors causing change in pride size are examined in the same way. Reproduction is profoundly influenced by behavioural factors, and to some extent by food; cub mortality is strongly influenced by both. However, these appear to be relatively unimportant in population regulation since in practice whatever the levels of these factors, more than enough female cubs survive to replace adult losses, and only some are recruited, the rest being expelled. This recruitment is influenced possibly by food and certainly by behaviour, and appears to be density dependen