The behaviour of six species of African schistosomes and their hybrids has been compared qualitatively and quantitatively in baboons, with a view to finding a schistosome of high infectivity but low pathogenicity to the baboon for use in heterologous immunity experiments. This study, which includes the first comparative study of the pathogenicity of these six species in primates, is complementary to earlier studies in rodents in which these schistosomes and their hybrids were compared in terms of egg morphology, snail infectivity, and infectivity and pathogenicity. In rodents, statistically significant differences were detected in the behaviour of each pair of sibling species with regard to percentage worm recovery, rate of egg production and percentage distribution of the eggs in the tissues, and the hybrids also differed significantly from their “parental” species.

Even greater differences were found in baboons. For example, although all the strains studied produced mature egg-laying adults in rodents, no worms, eggs, or histopathological changes resulted from exposure of baboons to S. rodhaini or S. bovis. With regard to prepatent period, S. mansoni was the shortest (5–6 weeks), S. haematobium the longest (12–14 weeks), while S. mattheei and S. intercalatum occupied intermediate positions (7–9 weeks). The hybrids all matured in 7–9 weeks except the S. bovis/S. mattheei hybrid (11–16 weeks). S. haematobium similarly had a much longer maturation time in rodents. Complete or partial “self-cure” occurred in 3 baboons but was not observed in rodents. The percentage worm recovery of S. mansoni was higher than for the S. mansoni/S. rodhaini hybrids while with the S. haematobium group of species, worm recovery was highest with S. mattheei followed by S. intercalatum and S. haematobium, the same order as in rodents. With the S. haematobium-group hybrids, worm recovery was generally high except with the S. bovis/S. haematobiuun hybrid.

1. Ninety-six lesser black-backed gulls, Larus fuscus L., captured at about 6 weeks of age, were kept in captivity for up to 2⅛ years. At intervals some gulls were killed and examined for helminth parasites, of which 14 species were found comprising 4 species of Trematoda, 5 species of Cestoda and 5 species of Nematoda.

2. Nine of the 14 species are commonly found in Holarctic gulls and may be regarded as characteristic members of the helminth fauna of these birds. Six species are recorded from L. fuscus for the first time.

3. The life span of different species of helminth parasites in L. fuscus varied from 2 months to 2⅛ years; few specimens lived in their host for more than one year. During captivity there was a marked decline in the variety of the helminth fauna of individual gulls, as well as a general decline in the incidence and intensity of infection with individual species of parasites. There was one exception, the tape-worm Tetrabothrius cylindraceus, which increased in incidence and maintained a fairly even intensity of infection in L. fuscus over 21 months. The results are discussed in relation to the ecology of the parasite and the host.