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Populations of two subspecies of Presbytis senex were studied for nearly two years in two different areas in Ceylon. Within the high density population of P. s. senex, social changes involving the replacement of the leader male and infant deaths were observed in five one-male troops. Evidence to indicate that such changes may also be occurring in the lower density population of the other subspecies, P. s. monticola, is given. A possible scheme of long-term population changes in these two subspecies is proposed and its applicability to several other langur species is discussed. Explanations for certain aspects of mother-infant relationship and territorial behaviour among langurs are suggested.

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