The herbicide 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone (pyrazon) was selective for sugar beets (Beta vulgaris L.). In preemergence treatments, sugar beets tolerated pyrazon at rates up to 6 lb/A, but at this level some mortality occurred. On the other hand, some weed species were susceptible to levels as low as 2 lb/A. Between emergence and the cotyledon stage, both beets and weeds were quite susceptible and the phytotoxicity of pyrazon increased with increasing temperature. In the late cotyledon stage, beets were resistant to pyrazon in aqueous suspension and partially susceptible to an oil-water suspension. Beets in the two to four-leaf stage tolerated an oil-water herbicidal combination, while lambsquarters (Chenopodium albium L.) and redroot pigweed (Amaranthus retroflexus L.) remained susceptible up to the six to eight-leaf stage. Rape (Brassica napus L.) was of intermediate susceptibility. In general, preemergence treatments were the most promising but could be replaced by postemergence treatments of pyrazon in an oil-water emulsion.