Metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H)one] provided adequate preemergence weed control on direct-seeded or transplanted tomato (Lycopersicon esculentum Mill.) on Plainfield sand and Drummer silty clay loam soils. Metribuzin demonstrated selective postemergence activity in the field but in glasshouse studies tomato cultivars had a wide range of tolerance to metribuzin applied to the foliage. Tomatoes were more susceptible to injury when they were less than 10-cm tall. Metribuzin was more effective in controlling broadleaf weeds than grass weeds, but grass control was acceptable with higher rates. Ultraviolet light decomposed metribuzin and weed control was improved when metribuzin was incorporated into the soil after application. Metribuzin treatment increased fruit yields. No metribuzin activity was found in the plots by the end of the growing season.