A method for the determination of arsenic species in marine samples using high performance liquid chromatography coupled to inductively coupled mass spectrometry (HPLC-ICP-MS) has been developed. Cation exchange HPLC with gradient elution using pyridine formate as the mobile phase was employed for the separation of a large number of arsenicals that occurred in the samples. The arsenic species were extracted using a 50% (v/v) methanol-water mixture and mechanical agitation overnight. The effect of the sample matrix on HPLC retention time was investigated and showed a dramatic effect for arsenobetaine and dimethylarsinoylacetic acid, whereas the cationic arsenocholine ion and tetramethylarsonium ion were not affected. The accuracy of the method for DMA, AsB and TMAs was validated with the CRMs DORM-2 and BCR626 Tuna. The concentrations found for arsenobetaine, dimethylarsinic acid and tetramethylarsonium ion were within the certified limits and low detection limits of 0.002-0.005 mug g(-1) dry mass (as As) for the different arsenic species were obtained. At least 23 different organic arsenic species were detected in a scallop kidney in one analytical run of 25 min duration. The ability of our analytical method to detect that many species simultaneously is useful for the study of the distribution and of the metabolic pathways of arsenic species in marine samples.