A highly selective and sensitive method for the determination of 30 meta- and para-substituted alkylphenols from phenol (C0) to nonylphenol (C9) in biota is described. Dichloromethane extracts of spiked cod liver and muscle samples are cleaned up by gel permeation chromatography, derivatised with pentafluorobenzoyl chloride and analysed by gas chromatography-mass spectrometry with negative-ion chemical ionisation. Quantification is done with isotope dilution of five internal standards of different chain length. The detection limits were in the low microg/kg levels. There were encountered problems with background levels of 4-nonylphenol. 4-Nonylphenol isomers were found in a number of plastic and rubber products used in the laboratory.