Four species of boreal conifer seedlings used for reforestation in eastern Canada were tested for their tolerance to the microbially produced herbicide bialaphos under controlled conditions. The tolerance levels of white spruce (Piceaglauca(Moench) Voss), black spruce (Piceamariana(Mill.) B.S.R), Norway spruce (Piceaabies(L.) Karst.), and red pine (PinusresinosaAit.) were studied over two consecutive growing seasons using different morphological and growth criteria. Application rates were the equivalent of 0, 1.0, 1.5, 2.0, 2.5, and 3.0 kg active ingredient (bialaphos) per hectare in a 1400-L solution and were applied in July, August, or September. Survival rates were a linear function of the dose of bialaphos, but were greater than 95% for all treatments. Results indicated that there was no significant correlation between visible foliar injury and the amount of epicuticular wax on needles. There was also no significant correlation between visible foliar injury and initial seedling height. However, a significant negative correlation was observed between the degree of foliar injury and the relative growth rate following treatment. No additional foliar injury occurred during the second growing season. Doses of bialaphos that did not exceed the equivalent of 2.0 kg active ingredient per hectare had no effect on subsequent growth. Tolerance was greatest when the herbicide was applied in August and least when applied in July. These results, combined with the lower tolerance previously demonstrated for several species that compete with planted conifers in eastern Canada, suggest that bialaphos has a strong potential as an alternative to chemically synthesized herbicides for vegetation management in conifer plantations.