This study is the first investigation on the endoparasite status of free living eastern lowland gorillas. Reported are actual infection extensities of parasites of gorilla groups in the Kahuzi-Biega National Park in Zaire. Behaviour patterns influencing the epidemiology of parasites and the impact of endoparasites on the clinical health of the gorillas are also investigated. The results permit an assessment of the likely development of the parasite status of gorilla populations in a situation where their habitats are continuously decreasing and where proximity to human settlements increases. This knowledge is necessary in order to establish measures for the conservation of the endangered species well in advance of threatening situations. Faecal samples of 64 habituated gorillas of the Kahuzi-Biega National Park were regularly investigated for over six months. Also, faecal samples of the park personnel and their families were investigated twice. The prevalence of helminths ranged between those of mountain- and western lowland gorillas and showed a similar range of species. The eggs or larvae of five nematodes and one cestode and two flagellates were identified in the gorilla faeces. Most of the gorillas also excreted physiological entodiniomorphs, which serve digestive functions. Infection extensities increased with increasing age of the gorillas; this pattern differed between different parasite species. No signs of galactogenically transmitted helminths could be detected nor could an influence of spatial ranging and the feeding behaviour of mothers on the parasite status of their sucklings be established. Lactating gorilla females excreted three of six of the parasite species and all female gorillas two of the six species more frequently than male gorillas. Differences between sexes and an effect of the female reproduction status were apparent for the heteroxenous Anoplocephala gorillae and for "larvae" the transmission way of the latter remains unknown. Also, differences in extensities between climatic seasons as well as between seasons of different food plants of the gorillas were marked for Anoplocephala gorillae. A negative correlation was established between the lengths of daily travels between two consecutive nest sites and prevalences of parasites for four investigated gorilla groups. Gorilla groups whose ranging areas did overlap considerably excreted helminth development stages more frequently than groups with a lower overlap. Higher excretion rates also were observed for the two groups which were more frequently visited by tourists when compared with groups with fewer contacts with humans. As far as the identification of the helminth species was possible none of them was shared between gorillas and man. Transmission potential from man to gorillas of the shared potentially pathogenic protozoa Giardia lamblia though could not be ruled out. All parasites identified in the stool samples of people have been transmitted to gorillas or other non-human primates in captivity. Three protozoa species identified in the gorillas and Enteromonas hominis for free living gorillas are described for the first time. Parasite infections in wildlife can result in reduced vitality and reproduction success on the population level, even though no sign of disease can be detected in individual animals. Behavioural traits that may reduce the risk of infection with parasites for gorillas could be avoidance of old nestsites for more than 54 days, the characteristics of nest building and the predominant defecation behaviour, the covering of different dayjourney lengths according to climate and vegetation seasons and the feeding on Guanea longispicata vines. The proportion of changes in the faeces structure was more elevated when faeces contained eggs of Anoplocephala gorillae and "small" eggs. However, no single case of general illness was detected in the population during the entire period of investigation. This result points out that a balanced relationship between the limited range of parasite species at medium infection prevalence rates and the gorilla hosts still likely exists. The results can be transferred, with some restrictions, to the endoparasite situation of the eastern lowland gorillas in the Congo basin and to the highly endangered mountain gorillas which live in similar biotopes. This investigation should assist to design measures for the protection and conservation of this so far under-investigated gorilla subspecies whose survival is particularly threatened by the ongoing civil war in the area.