Investigations of the bionomics and populations of Cyzenis albicans (Fall.), in relation to its principal host, Operophtera brumata (L.), were carried out during 1962–1963 in two stands of red oak in Nova Scotia. C. albicans was well synchronized with its host and oviposited only on leaves damaged by the host or by other defoliators. C. albicans was found experimentally to develop in larvae of a native species, Operophtera bruceata (Hulst). The distributions per leaf cluster of parasite eggs and host larvae were of the negative binomial type and it sampling technique employing 12 leaf clusters per tree produced standard errors which varied between 18 and 33% of the mean for average densities of 12 to 2 eggs per cluster. Parasite eggs per leaf cluster and percentage parasitism increased as host density increased. An associated defoliator, Pseudexentera cressoniana Clem., because it apparently stimulated parasite oviposition but was not itself susceptible to parasitism, had a detrimental effect on the efficiency of the parasite.