This study investigates the demography and interspecific interactions of 6 Heterohyrax brucei and 4 Procavia johnstoni populations, which inhabited 6 kopjes (rock outcrops) in the Serengeti National Park, Tanzania over a period of 17 years. On three kopjes (H1, O2 and PH2) both species lived sympatrically, while on the other three (H2, H3 and P1) either one or the other species occurred allopatrically. The rainfall pattern between 1971/1972 and 1981/1982 had undergone major changes. There was significantly less precipitation in the wet season and the dry season had become extremer and longer. The kopje vegetation, recorded as percentage crown cover of browse from bushes and trees, had changed also, decreasing between 1971/72 and 1982 on 5 kopjes. In three kopjes (H1, O2 and PH2), which were shared by both hyrax species, an increase in the number of P. johnstoni, a decrease (H1 and O2) and extinction (PH2) of H. brucei took place between the time periods 1971-1976 ("good browse years") and 1982-1984 ("poor browse years"). These changes are probably the result of indirect interspecific competition (exploitative competition) for browse material, which is the main food source for both species during the dry season. When the browsing resources are abundant coexistence on sympatric kopjes between both species is possible, but when browse is scarce the result is competitive exclusion of H. brucei by P. johnstoni. It is also argued, that P. johnstoni by being the larger species and by also being a grazer had several ecological advantages over H. brucei. In kopjes H2, H3 and P1 where H. brucei occurred allopatrically the population increased (H2, P1) or decreased (H3) during the same time period. Natural extinction was observed in kopje P1 in the allopatrically living P. johnstoni group through mange. Dispersal of 7 females and 7 males H. brucei as well as 1 female and 4 male P. johnstoni were recorded in kopjes H1, H2 and P1. One female H. brucei that immigrated successfully into kopje H2 reached an age of over 11 years. One H. brucei family group (H2) shifted its birth season from December-January to May-August. Successful natural colonization was recorded for H. brucei in kopje P1 and for P. johnstoni in H1. Two kopjes (PH2 and H3), which had no hyrax and are located far away from other hyrax populations, were experimentally colonized in 1971 and 1972 respectively. The P. Johnstoni on PH2 and the H. brucei group on H3 had been under breeding isolation for 16 years. Under such conditions P. johnstoni males can reach an age of over 8.5 years. These long-term observations have shown that the occupancy of the kopjes by both species is a dynamic process depending on the combination of several abiotic (rainfall and availability of holes and hiding places) and biotic factors (interspecific and intraspecific competition for food, interspecific cooperation, predation and parasites) as well as the degree of inbreeding dependent on the geographic isolation of the kopje.