Squoxin (1,1'-Methylendi-2-napthol) was tested in laboratory and field bioassays to evaluate its biological activity to rainbow trout (Salmo gairdneri), cutthroat trout (Salmo clarki), Utah chub (Gila atraria), carp (Cyprinus carpio), reside shiner (Richardsonius balteatus), and mountain sucker (Catostomus platyrhynchus) under various conditions of water quality and temperature. The compound was toxic to all species and most effective from greatest to least to mountain sucker, reside shiner, Utah chub, cutthroat trout, and rainbow trout and carp. Selectively and safety indices for Utah chub compared with the two species of trout indicated a selective control potential for squoxin in hard water at 12.1C and very hard water at 5.6C and 18C. Efficacy of squoxin for selective control of Utah chub in trout waters was similar to the selective control of the sea lamprey (Petromyzon marinus) with 3-trifluoromethyl-4-nitophenol (TFM) in which selection of time and place precedes toxicant use. The toxicity of squoxin was affected more by water temperature than by different water hardnesses or pH. The cumulative effects of various biological, chemical, and physical factors in field tests significantly reduced the toxicity of squoxin to fish.