Two helminth communities, comprising seven species in Ochotona princeps and five in O. collaris, were studied for site specificity and interspecific interactions. There were no cases of competitive exclusion, but four species pairs were found concurrently more often than by chance, possibly due to similarities in life cycles. One case of interactive site segregation was demonstrated. There was a high degree of selective site segregation. The pinworm species superficially appeared to overlap,

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... closer examination demonstrated a migratory phenomenon which concealed a form of site selection related to gut lumen contents in addition to site. An attempt was made to relate the degree of helminth-helminth interaction to? the age of the helminth-host association. The communities are old and the component species have therefore been associated for many generations. Only the more recent community members were found to be interactive in their site segregation, indicating that interspecific interactions may play a role in the evolution of site specificity.