;Black Mexican Sweet' (BMS) maize (Zea mays L.) tissue cultures were selected for tolerance to sethoxydim. Sethoxydim, a cyclohexanedione, and haloxyfop, an aryloxyphenoxypropionate, exert herbicidal activity on most monocots including maize by inhibiting acetyl-coenzyme A carboxylase (ACCase). Selected line B10S grew on medium containing 10 micromolar sethoxydim. Lines B50S and B100S were subsequent selections from B10S that grew on medium containing 50 and 100 micromolar sethoxydim, respectively. Growth rates of BMS, B10S, B50S, and B100S were similar in the absence of herbicide. Herbicide concentrations reducing growth by 50% were 0.6, 4.5, 35, and 26 micromolar sethoxydim and 0.06, 0.5, 5.4, and 1.8 micromolar haloxyfop for BMS, B10S, B50S, and B100S, respectively. Sethoxydim and haloxyfop concentrations that inhibited ACCase by 50% were similar for BMS, B10S, B50S, and B100S. However, ACCase activities were 6.01, 10.7, 16.1, and 11.4 nmol HCO(3) (-) incorporated per milligram of protein per minute in extracts of BMS, B10S, B50S, and B100S, respectively, suggesting that increased wild-type ACCase activity conferred herbicide tolerance. Incorporation of [(14)C]acetate into the nonpolar lipid fraction was higher for B50S than for BMS in the absence of sethoxydim providing further evidence for an increase in ACCase activity in the selected line. In the presence of 5 micromolar sethoxydim, [(14)C]acetate incorporation by B50S was similar to that for untreated BMS. The levels of a biotin-containing polypeptide (about 220,000 molecular weight), presumably the ACCase subunit, were increased in the tissue cultures that exhibited elevated ACCase activity indicating overproduction of the ACCase enzyme.