Infection with the cestode Spirometra spp. was studied in 2 populations of lions in the Serengeti and the Ngorongoro Crater in Tanzania, East Africa. These 2 lion populations lived in different habitats and were known to differ genetically: lions in the Serengeti were outbred, whereas lions in the Ngorongoro Crater were inbred. Faecal samples were collected from 112 individually known lions between March 1991 and November 1992. Over 60% of lions were infected and the median intensity of infection was 975 eggs per g of faeces. The distribution of egg counts was overdispersed. There was variability through time, though this was unrelated to seasons delimited by rainfall. There were no significant differences in levels of infection between age classes; cubs less than 9 months were already heavily infected. Sex and reproductive status did not have a significant effect. However, there were significant differences in intensities of infection between the Crater and the Serengeti populations – Spirometra spp. showed a higher level of infection intensity in the Crater population – with some variation between prides within these populations. Allozyme heterozygosity scores were available for a subset of 28 lions but were unrelated to levels of Spirometra infection. It was not possible to ascribe differences in levels of parasite infection to genetic rather than ecological factors.