Preemergence herbicidal activities of 27 3-phenoxypyridazine derivatives were evaluated in pots under upland and submerged conditions. Each of the compounds containing an alkyl or halogen at the ortho position of the benzene ring showed marked herbicidal effects. The activity of some disubstituted compounds carrying one of the substituents at the ortho position was the next. Among the compounds tested, 3-(2-methylphenoxy)-, 3-(2,3-dimethylphenoxy)-, 3-(2,4-dimethylphenoxy)-, and 3-(2,6-dimethylphenoxy)-pyridazines exhibited an excellent margin of selectivity for tomato (Lycopersicum esculentum Mill.), cotton (Gossypium hirsutum L.), soybean (Glycine max (L.) Merr.), and Azuki bean (Phaseolus angularis Wight). For transplanted rice (Oryza sativa L. ‘Norin No. 29′) under submerged conditions, 3-(2-isopropylphenoxy)-and 3-(2-n-butylphenoxy)-pyridazines showed a remarkable margin of selectivity, though the herbicidal activity of both compounds was slightly inferior to that of 3-(2-methylphenoxy)-pyridazine.