Big-vein disease occurs on lettuce worldwide in temperate regions. The causal agent is transmitted from diseased to healthy plants by zoospores of the lettuce root-infecting fungus Olpidium brassicae. To develp chemical control measures for lettuce big-vein disease, the efficacy of thiophanate-methyl was confirmed in a commercial field. Soil drenching with thiophanate-methyl has been found to be effective over a long period of time even in fields severely infested with the disease. Moreover, combining the application of the chemical with the cultivation of a moderately resistant cultivar reduced the rate of diseased plants and improved the marketability of crops. The chemical was effective even in winter cultivation, when economic damage predominates.