The cellular localization of difenzoquat was investigated using suspension cell cultures derived from inbred wild oat (Avena fatua L.) lines that were susceptible (S) or resistant (R) to the herbicide. Compartmental analysis of S cell cultures resolved [3-14C]difenzoquat efflux from three cellular compartments which contained 75, 19, and 6%, respectively, of the total absorbed difenzoquat. Greater than 98% of the efflux from S cells occurred within 420 min. In contrast, compartmental analysis of R cell cultures differentiated only two cellular components, containing 73 and 27% of the absorbed difenzoquat. Further, difenzoquat efflux from R cell cultures occurred more slowly, requiring >2000 min for 98% efflux to be achieved. Washing cell cultures preloaded with [3-14C]difenzoquat in methanol:chloroform indicated that difenzoquat was preferentially bound in cell wall material of R cell cultures. In whole plants, a greater proportion of total leaf-absorbed [3-14C]difenzoquat was present in an insoluble residue in R plants than in S as determined by tissue homogenization and liquid scintillation counting. By 96 h after treatment with [3-14C]difenzoquat, >90% of the absorbed radiolabel was extractable from S plants, whereas <10% was extractable from R tissues. The results indicate that tight binding of difenzoquat in R cell walls may be responsible for difenzoquat resistance in wild oats.