The increase in dry weight during the culture of bean callus cultures was inhibited by the herbicide dichlobenil (2,6-dicholorobenzonitrile) with an I(50) of 0.5 µM. However bean calli became tolerant to a concentration of 12 µM by a stepwise increase in the concentration of the inhibitor in each subculture. Tolerant calli growing in 2,6-dicholorobenzonitrile developed with hollow protuberances. Groups of cells in these protuberances had irregular cell walls surrounded by a thicker cell wall with a lamellate structure and without a differentiated middle lamella. FTIR spectra of tolerant cell walls revealed an increase in both esterified and non-esterified pectins. Cell wall fractionation showed that in tolerant cell walls the xyloglucan-cellulose network of non-tolerant cell walls was partly replaced by a pectin-rich network mainly formed of cross-linked polyuronides with a large proportion of homogalacturonan. These modifications are comparable to those described for bean calli tolerant to isoxaben, pointing to a related mechanism of tolerance for both herbicides.