The larch sawfly, Pristiphora erichsonii (Hartig), is the most important pest of tamarack (Larix laricina) in North America. Entomologists differ as to whether this species is an introduction from Europe or a native of America. The available evidence appears to indicate that the insect was introduced from Europe and that it has probably been present in North America since about the beginning of the nineteenth century... A number of devestating outbreaks of the insect have been recorded in North America since 1882... The most recent and continuing outbreak began in Manitoba about 1938 and now covers extensive areas in Saskatchewan, Manitoba, Ontario and the Lake States... An important phase of the problem that has been studied intensively concerns the effectiveness of Mesoleius tenthredinis Morley, an introduced parasite of the larch sawfly. It will be shown in a succeeding section that in the past this parasite must have been an effective controlling factor in western and central Canada. Studies and observations undertaken since 1940 have revealed that the larch sawfly in Manitoba and Saskatchewan has evidently developed an effective immunity to the parasite. The author conducted an investigation of the nature of this immunity from 1949 to 1951 and this paper gives an account of the results of this study. Research has been conducted in Riding Mountain National Park, The Whiteshell Forest Reserve, and Winnipeg, Manitoba and at Prince Albert, Saskatchewan. Since M. tenthredinis has decreased in effectiveness in Manitoba and Saskatchewan in past years it is important to understand the reasons for this decrease in order to properly appraise the possibility of remedying the situation. In British Columbia M. tenthredinis is still an effective parasite of the larch sawfly and it is important to know what to watch for when sampling the larch sawfly population in British Columbia in the future in order to determine whether or not this parasite is maintainaing its effectiveness in that province.