Different sub-sampling procedures were applied for the determination of mercury species (as total mercury Hg, methylmercury MeHg+ and inorganic mercury Hg2+) in frozen fish meat. Analyses were carried out by two different techniques. After the sample material was pre-treated by microwave digestion, atomic fluorescence spectroscopy (AFS) was used for the determination of total Hg. Speciation analysis was performed according to the following procedure: dissolution of sample material in tetramethylammonium hydroxide (TMAH), derivatisation with sodium tetraethylborate (NaBEt4), extraction into isooctane and measurement with gas chromatography inductively coupled plasma mass spectrometry (GC-ICPMS) for the identification and quantification of methylmercury (MeHg+) and inorganic mercury (Hg2+). The concentration range of total Hg measured in the shark fillets is between 0.9 and 3.6 microg g(-1) thawed out shark fillet. Speciation analysis leads to > or =94% Hg present as MeHg+. Homogeneity, storage conditions and stability of analytical species and sample materials have great influence on analytical results. Sub-sampling of half-frozen/partly thawed out fish and analysis lead to significantly different concentrations, which are on average a factor of two lower.