In the greenhouse, the methyl ester of diclofop {2-[4-(2,4-diclorophenoxy)phenoxy] propanoic acid} as a postemergence application at 1.1 kg/ha ai did not affect the growth of the seven dicotyledonous crops with the exception of slight injury to yellow [white] mustard (Brassica hirta Moench). Of the 13 gramineous species, corn (Zea mays L.), green foxtail [Setaria viridis (L.) Beauv.], oats (Avena sativa L.) wild oats (Avena fatua L.), sorghum [Sorghum bicolor (L.) Moench], and timothy (Phleum pratense L.) were rated highly susceptible, while barley (Hordeum vulgare L.), bromegrass [smooth brome] (Bromus inermis Leyss.), durum wheat (Triticum durum Desf.), intermediate wheatgrass [Agropyron intermedium (Host) Beauv.], Russian wild ryegrass (Elymus junceus Fisch.), triticale (X Triticosecale Wittmack), and wheat (Triticum aestivum L.) were relatively tolerant. Residue of diclofop in the soil partially controlled green foxtail which was seeded 3 weeks after herbicide application. Whether applied to wild oat shoots or roots, diclofop affected the entire plant, but growth reduction was great when applied to the shoots. Field results confirmed that foliar application gave the best weed control resulting in larger wheat yield increases. Wild oat control was greatly enhanced when an adjuvant was added to diclofop. This enhanced herbicidal activity appeared to have no effect on tolerance of barley at 0.8 kg/ha nor wheat at 1.1 kg/ha. Increasing the temperature from 12 to 28 C decreased wild oat control from diclofop at 1.1 kg/ha. In the field, diclofop at rates ranging from 0.8 to 2.2 kg/ha gave good control of wild oats and green foxtail resulting in significantly increased wheat yield compared to the checks. However, barley yield increases were obtained only at the rates from 0.8 to 1.1 kg/ha, indicating that at the higher rates barley was less tolerant than wheat. The 0.6 kg/ha rate with an adjuvant gave adequate weed control and significant barley yield increases over the checks, when evaluated on five barley cultivars.