A procedure for the extraction and determination of methyl mercury and mercury (II) in fish muscle tissues and sediment samples is presented. The procedure involves extraction with 5% (v/v) 2-mercaptoethanol, separation and determination of mercury species by HPLC-ICPMS using a Perkin-Elmer 3 Î¼m C8 (33 mm Ã¿ 3 mm) column and a mobile phase 3 containing 0.5% (v/v) 2-mercaptoethanol and 5% (v/v) CH3OH (pH 5.5) at a flow rate 1.5 ml minâ¿¿1 and a temperature of 25 Â°C. Calibration curves for methyl mercury (I) and mercury (II) standards were linear in the range of 0â¿¿100 Î¼g lâ¿¿1 (r2 = 0.9990 and r2 = 0.9995 respectively). The lowest measurable mercury was 0.4 Î¼g lâ¿¿1 which corresponds to 0.01 Î¼g gâ¿¿1 in fish tissues and sediments. Methyl mercury concentrations measured in biological certified reference materials, NRCC DORM â¿¿ 2 Dogfish muscle (4.4 Â± 0.8 Î¼g gâ¿¿1), NRCC Dolt â¿¿ 3 Dogfish liver (1.55 Â± 0.09 Î¼g gâ¿¿1), NIST RM 50 Albacore Tuna (0.89 Â± 0.08 Î¼g gâ¿¿1) and IRMM IMEP-20 Tuna fish (3.6 Â± 0.6 Î¼g gâ¿¿1) were in agreement with the certified value (4.47 Â± 0.32 Î¼g gâ¿¿1, 1.59 Â± 0.12 Î¼g gâ¿¿1, 0.87 Â± 0.03 Î¼g gâ¿¿1, 4.24 Â± 0.27 Î¼g gâ¿¿1 respectively). For the sediment reference material ERM CC 580, a methyl mercury concentration of 0.070 Â± 0.002 Î¼g gâ¿¿1 was measured which corresponds to an extraction efficiency of 92 Â± 3% of certified values (0.076 Â± 0.04 Î¼g gâ¿¿1) but within the range of published values (0.040â¿¿0.084 Î¼g gâ¿¿1; mean Â± s.d.: 0.073 Â± 0.05 Î¼g gâ¿¿1, n = 40) for this material. The extraction procedure for the fish tissues was also compared against an enzymatic extraction using Protease type XIV that has been previously published and similar results were obtained. The use of HPLC-HGAAS with a Phenomenox 5 Î¼m Luna C18 (250 mm Ã¿ 4.6 mm) column and a mobile phase containing 0.06 mol lâ¿¿1 ammonium acetate (Merck Pty Limited, Australia) in 5% (v/v) methanol and 0.1% (w/v) l-cysteine at 25 Â°C was evaluated as a complementary alternative to HPLC-ICPMS for the measurement of mercury species in fish tissues. The lowest measurable mercury concentration was 2 Î¼g lâ¿¿1 and this corresponds to 0.1 Î¼g gâ¿¿1 in fish tissues. Analysis of enzymatic extracts analysed by HPLC-HGAAS and HPLC-ICPMS gave equivalent results.