Mutants resistant to the herbicide N-(3-[1-ethyl-1-methylpropyl]-5-isoxazolyl)-2,6,dimethoxybenzamide (isoxaben) were recovered from an M2 population of Arabidopsis thaliana. Two of these mutants, DH47 and DH48, had a high level of resistance in the homozygous state. Crosses of these mutants to marker strains, and to each other, showed that each contained a mutation at a single locus tightly linked to lutescens, a marker on the fifth chromosome of A. thaliana. Growth curves of these mutants and of the F1 progeny of a cross with the wild type parent strain, in the presence of different concentrations of the herbicide, showed that both mutants display a semidominant phenotype. The two mutations differed in their degree of resistance, both as homozygotes and heterozygotes. This suggests that they are two different alleles. Callus cultures were established from plants homozygous, as well as heterozygous, for each of these mutations. Growth curves of these cultures in the presence of the herbicide mimicked the data obtained in vivo indicating that sensitivity to isoxaben is not dependent on a differentiated function.