Detailed genetic analysis have provided valuable information about the population structure of several commercially important fish species. The investigations have mainly concentrated on the problem on stock identification, and recently to the concervation of genetic resources. The method of enzyme electrophoresis utilize small pieces of fish tissue which can easily be obtained during the biological sampling of fish populations. In tagging experiments, however, it has been impossible to get tissue samples for genetic analysis. For this reason, a biopsi apparatus was constructed which permit one to take small pieces of white muscle from alive fish. The apparatus is described below. The biopsi apparatus was tested on a tagging cruise in the Møre area during the spawning season of cod. Sampling of white muscle of cod was easily to perform, and the fish were tagged in the conventional manner afterwards. The samples taken by biopsi were frozen and later analysed in the laboratory for the polymorphic enzymes phosphoglucose isomerase (PGI), phosphoglucomutase (PGM), lactate dehydrogenase (LDH), isocitrate dehydrogenase (IDH) and a-glycerophosphate dehydrogenase (GPD) . The enzyme activities in the samples were high, and thus permitting classification of each individual fish to enzyme phenotype. About 600 fish were both tagged and classified to phenotype for most of the enzymes in question. Comparisons of phenotypes and allel frequencies indicated heterogenity between different catches during the cruise. Some catches, mainly consisting of immature cod individuals, differ significantly from the others. Preliminary data on recapture of tagged fish, suggest that this fish belong to cod occurring in coastal sea water. The material, however, was limited and more data are needed to get a better knowledge of the population structure of cod in the Møre area.