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 (Delta 9-cisC(16:1)) and oleic (Delta 9-cisC(18:1)) acids. The decreased transcription of the OLE1 gene (encoding the Delta 9 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. (c) 2005 Elsevier Inc. All rights reserved.