During the years 1981-1994, Black-nest Swiftlets Collocalia maxima were found breeding in small numbers (65 nests in 1994) for the first time in Vietnam. The nests were scattered among the much more numerous nests of the White-nest Swiftlet Collocalia fuciphaga germani in caves in islands along the coast of the Khanh Hoa province. In Vietnam, C. maxima has a diet constituted by 80% Hymenoptera. It starts breeding about two weeks earlier than C. fuciphaga, but its young leave the nests at about the same time as the ones of this species. Breeding production is higher in C. maxima than in M. fuciphaga (0.75 vs 0.4-0.6 fledglings/pair/year), but population growth is slower. C. maxima continues nest-building during incubation, and there are almost no differences between first nests and nests built after the first ones have been harvested. In Vietnam, contrary to adjacent regions of South-east Asia, there is a distinct seasonality in nesting and moulting cycles of C. maxima.