Field trials were conducted at Portage la Prairie (1982 and 1983) to determine the effects of increasing densities (0, 7.5, 15, 30, 45 and 60/m2) of volunteer wheat (Triticum aestivum) or volunteer barley (Hordeum vulgare) on the growth and yield of flax (Linum usitatissimum) or canola (Brassica napus). Constant densities of wild oats (Avena fatua) at 30-35/m2 and green foxtail (Setaria viridis) at 150-180/m2 were also seeded to simulate a weed flora which might occur under normal farming practices. The graminaceous weeds were selectively removed with herbicides in order that crop growth and final yield could be assessed in the presence and absence of weeds. Both the shoot vegetative dry weight and the seed yield of flax and canola were increasingly reduced as volunteer density increased. The greatest incremental reduction in oilseed yield occurred between the weed-free situation and the first density increment. Volunteer barley was more competitive than wheat in both crops. At densities of only 15 volunteer barley plants/m2 the yield of flax was reduced by 35% and 44% (1982 and 1983, respectively), significantly higher than comparable reductions of 12% and 14% (1982 and 1983, respectively) recorded in canola. The accuracy of predictive assessments of the potential yield reductions caused by volunteer barley or wheat was influenced by the presence of mixed weed populations (including green foxtail and wild oats) and the weather throughout the season. The results obtained could be used to determine the cost/benefit relationship where known volunteer infestations are to be selectively controlled in crops by graminicides.