The hippopotamus population along the Runde River in Gonarezhou National Park, Zimbabwe, attained a density in the late 1970s which threatened vegetation resources of this semi-arid park. A sample of 105 hippopotami taken in 1978 was used to determine the population's age and sex structure, and reproductive characteristics. An average school consisted of 11.4 individuals, comprising 2.2 adult bulls, 4.6 adult cows, 2.2 and 1.4 immature males and females, respectively, and 0.8 calves. Females constituted 54% of the population, and 58% of foetuses; neither differed from parity. Testes mass and seminiferous tubule diameter increased with age according to a logistic function. Puberty commenced at about 4 years, and sexual maturity at 8–8.3 years, respectively at 47% (655 kg) and 65% (1070 kg) of mean adult male mass. Males were physically mature by 17.5 years. Based on the development of ovary mass, uterus mass, and uterine horn length of non-pregnant or pregnant females, the pubertal interval was 24 months commencing at 7 years when body mass reached 71% of mean adult female mass (964 kg), and sexual maturity commenced when body mass reached 78% of mean adult female mass (1050 kg). Females attained physical maturity at 13 years. Conceptions occurred in seven of eight sampled months, with most occurring in June and peak parturition estimated to occur in February. The pregnancy rate of sexually mature females was 36.8%, the incidence of pregnancy was 0.56 calves/female/year, anoestrus was 13.7 months, post-partum oestrus was 5.7 months, and the calving interval was 21.7 months. Calf mortality (<2 years) was 42%. Female births per female of reproductive age was 0.092. Reproductive characteristics of hippopotamus across Africa illustrate their plasticity in relation to climatic variation and population density, suggesting populations may be regulated without any need for management intervention.