Species-specific isotope dilution (ID) in combination with gas chromatography-inductively coupled plasma-mass spectrometry (GC-ICP-MS) for separation and determination of methylmercury (MeHg) and inorganic mercury (Hg) in fish tissue and Zooplankton is described. Samples were digested with methanolic potassium hydroxide. Analytes were derivatized with sodium tetraethylborate and absorbed in an isooctane trap. ID analysis was performed using an enriched spike of ²⁰¹Hg and a laboratory synthesized enriched spike of Me Hg. The precision of repeated measurements was always less than 5%. The absolute detection limits obtained with GC-ICP-MS for MeHg and Hg were 2.8 ng and 4.6 ng, respectively. Analytical precision was typically better than 5% over ten repeated measurements. The accuracy of the method has been validated with certified reference materials (DORM-2 and BCR 414). The total mercury concentration results of the real samples as well as the CRM have been compared with those obtained by cold vapor-high resolution ICP-MS using ID approach.