Cod (Gadus morhua L.) were caught in pond nets at different locations in Danish coastal waters and examined for the presence of nodules in the internal organs. Cod with nodules were found at two carbohydrate-contaminated locations with prevalences up to 20%. Acid-fast bacteria were in many cases observed in the nodules. A procedure for the isolation of mycobacteria is described. The isolation method was developed by investigations of Mycobacterium marinum and Mycobacterium fortuitum injected in rainbow trout (Oncorhynchus mykiss). Fish tissue was decontaminated with sodium hydroxyde and oxalic acid and isolation was done on Löwenstein-Jensen Medium. Applying this method to naturally infected cod, successful isolation of mycobacteria was carried out. Growth conditions and biochemical characteristics of mycobacteria isolated from fish are discussed.