Rhizoctonia solani Kuhn is the causal agent ofthe sheath blight of rice (Orysa saliva L.), one of the most important rice diseases in Colombia. G1yphosate (N-phosphonomethyl glicine acid) is sprayed in rice fields as a preplant herbicide, specially aimed at red rice (O. saliva). Preliminary field observations in Tolima, Colombia suggested a relationship between intensive use of glyphosate and sheath blight incidence. Thus, the present study was undertaken to shed light on the effect of glyphosate on R. solani and Thrichoderma sp., a.putative antagonist, under laboratory conditions. The effect of the herbicide was assessed on the growth ofR. solani in summerged culture and solid media and of Thrichoderma on solid media. R. sotani withstood up to 300 mg L-! without expressing reduction in growth, Increasing concentrations of the herbicide caused growth reduction, and the detrimental effect reached a plateau at 2500 mg L-1. Rhizoctonia growth was not enhanced at any herbicide concentration. The expected concentration of glyphosate in the soil after spraying at commercial dosages would be O, 75 mg Kg', which is well below the observed in vitro threshold. The response of Thrichoderma was similar to that of R. sotani. Glyphosate did not affect the antagonistic ability of Thrichoderma against R. solani. These results do not support the view that glyphosate, as an herbicide treatment in rice field, may affect in any way the population densities of R. sotani or its antagonist Thrichoderma sp. Therefore, the postulated connection between intensive herbicide use and increased sheath blight incidence is not substantiated on mycelial growth alone. This relationship should be explored in terms of selerotial survival and germination.