Cocoons of Neodiprion sertifer (Geoff.) were collected in southwestern Ontario in 1941, 1943, 1946, 1947, and 1949 for experimental investigations at the Belleville laboratory. A total of 8,326 cocoons were collected, mainly within about eight miles northeast and east of Sarnia, Ont., but a few were collected at Strathroy and Brigden, Ont. Each cocoon was incubated separately in a small vial so that each parasite could be associated with the host cocoon from which it emerged. Sawflies emerged from 68 per cent of the cocoons and parasices from 12 per cent; dissection of the remaining cocoons showed 16 per cent contained dead sawfly larvae and pupae, and four per cent dead parasite larvae.