In investigations in Yugoslavia covering several years, 4th-instar and older Imae of Neodiprion sertifer were taken from pine [Pinus] forest during a period of about 12 days in May and on the same day were allowed to feed on material infected with nuclear polyhedrosis virus at a concn of about 10000000 polyhedra/mi. Mortality reached 49-56%. Numbers of larvae surviving were greatest among those collected early, when over 7 1 % of the adults emerging from the cocoons were males. The adults emerging from the smaller numbers of cocoons spun by later-wuected (i.e. older) larvae were predominantly female, indicating that at the time of collection the males have ceased feeding and thus avoid infection. A proportion of cocoons gave nse to Lantachus eques and (in greater numbers) Lophyroplectus luteator in the following spring. Many more of the parasitoids emerged from late- dm from earlier-cohected material. Dead larvae and spun cocoons were lighter in the case material collected earlier. Overall, the data indicated differences in the sex of the larvae according to the collecting time. none