Yeast cells adapted to the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) exhibit a plasma membrane less susceptible to 2,4-D-induced disruption and are more tolerant than unadapted cells to lethal concentrations of the herbicide. These cells, adapted to grow in the presence of increasing concentrations of 2,4-D, were found to exhibit a dose-dependent increase of the saturation degree of membrane fatty acids, associated to the higher percentage of stearic (C(18:0)) and palmitic (C(16:0)) acids, and to the decreased percentage of palmitoleic (Delta9-cisC(16:1)) and oleic (Delta9-cisC(18:1)) acids. The decreased transcription of the OLE1 gene (encoding the Delta9 fatty acid desaturase that catalyses the conversion of palmitic and stearic acids to palmitoleic and oleic acids, respectively) registered in 2,4-D adapted cells suggests that yeast adaptation to the herbicide involves the enhancement of the ratio of saturated (C(16:0) and C(18:0)) to monounsaturated (C(16:1) and C(18:1)) membrane fatty acids through a reduced OLE1 expression.