Microcystins (MCs) are widespread cyanobacterial toxins in freshwater systems, and have been linked to both acute and chronic health effects. A growing number of studies suggest that MC can bioaccumulate in food webs. Although, several methods (i.e. ELISA, LC-MS) have been developed for analysis of MC in water, extraction (for subsequent analysis) of the toxin from biological matrices (i.e. animal tissues) is impeded owing to covalent binding of toxins and active sites of their cellular targets, i.e. protein phosphatases. As an alternative approach, chromatographic methods for analysis of a unique marker, 2-methyl-3-methoxy-4-phenylbutanoic acid (MMPB), the product of the Lemieux oxidation of MCs, have been previously developed, and shown to measure total (bound and unbound) MC. Application, however, has been limited by poor recovery of the analyte. An improved recovery method is proposed - specifically the use of solidphase microextraction (SPME). The MMPB analogue, 4-phenylbutanoic acid (4PB), and oxidized MC, were used to develop methods, and we specifically investigated several SPME fibres, and post-oxidation steps. Specifically, a method employing post-oxidation methyl esterification, followed by headspace SPME recovery of MMPB, was developed, and subsequently applied to analysis of environmental samples (i.e. fish tissues) previously shown to contain MCs. The method shows high linearity for both water and tissues spiked with MC, and an improved limit of quantitation of approximately 140 ng g(-1). Evaluation of field samples by SPME-GC/MS detected considerably higher levels of MC, than detected by conventional methods (i.e. ELISA), and it is proposed that this technique reveals MC (particularly in the bound form) that is not detected by these methods. These results indicate that the developed method provides improved detection capability for MC in biological matrices, and will enhance our ability to understand bioaccumulation in freshwater food webs, as well as monitor exposure.