The population size of common hippopotamus, Hippopotamus amphibious in the Luangwa valley, Zambia was assessed for the period of 1976 to 2008. A comparison of historical data on population size for the period of 1952 to1975 was also made. Ecological carrying capacity (K) and carrying capacity band were estimated from data on population size. The river bank foot count method was used, which involved the counting of individuals and taking Global Positioning System (GPS) locations of hippopotamus schools. Ecological Carrying capacity (K) was 6,000 hippopotami and density at K was 35/km. The population was found to be oscillating within carrying capacity band of 2,000 individuals in eight (8) irregular cycles. From K, annual population changes were small and insignificant. A weak correlation was found between population size against time, suggesting a slow down in population growth from 1976 to 2008 (R2 = 0.22). The highest population size of 6,832 hippopotami and density of 42 was reached in 1984. The lowest was 4,765 hippopotami and density of 29/km recorded in 1978. Within the period of 1976 to 2008, the population oscillated between 5,000 and 7,000 individuals, which were symptomatic of a population at K. The slow down in population growth was attributed to environmental resistance. More studies are required to identify factors responsible for environmental resistance which is caused by the rise and fall in the hippopotamus population of the Luangwa River.