The measurement of trace elements and phosphorus in marine biological tissues by ICP-MS after low-volume closed-vessel microwave digestion with nitric acid is described. Variations in final nitric acid concentration (0-6%, v/v) of digests caused a decrease in ICP-MS signal only for Zn and Se. Polyatomic interferences from Ca, Na, K, Mg, P, S, Cl, and C at levels expected in marine tissues were investigated. Isotopes of Cr, V, Fe, Co, Ni, Cu, Zn, Cd, As, and Se are likely to suffer from interferences. Choosing alternative isotopes for most elements can minimize potential interferences. Eleven CRMs were analyzed: BCR CRM 414 Plankton, IAEA 140/TM Fucus, NIES No. 9 Sargasso Seaweed, IAEA-MA-A-1 Copepods, NIST SRM 1566a Oyster Tissue, AGAL-3 Prawn, NRCC Tort-1 Lobster Hepatopancreas, AGAL-2 Shark, IAEA MA-A-2 Fish Flesh, NRCC DOLT-1 Dog Fish Liver, NIST CRM 50 Albacore Tuna. Acceptable concentrations were obtained for P, V, Mn, Fe, Cr, Co, Ni, Cu, Zn, As, Se, Ag, Cd, and Pb. There was little indication of - interference from CI on As and Se or from Ca on Co. Sb concentrations were in general agreement with certified concentrations, but variability in certified concentrations are large, thus not allowing a rigorous comparison to be made. Cr concentrations are also in general agreement with certified concentrations when isotopic mass counts are corrected for carbon and/or chloride interferences. Hg values are consistently overestimated and variable as the CRM concentrations are near the instrumental limit of detection.