Salmonids freeze to death if they come into contact with ice. Many marine fish species that inhabit icy sea waters synthesize antifreeze proteins (AFP) to protect them from freezing. Production of stable lines of freeze-resistant salmon and other species would greatly facilitate development of sea-pen aquaculture in many regions. We successfully introduced winter flounder AFP genes into Atlantic salmon. Research to date indicates stable genomic integration and low levels of expression of winter flounder AFP genes in a small number (approximately 3%) of salmon developed from microinjected eggs. Inheritance of the AFP gene by offspring (F1) from crosses between transgenic and wild-type salmon revealed that the transgenic flounders (F0) were germ-line mosaics. Low levels of AFP precursors could be detected in the blood of all these transgenic offspring (F1). Approximately 50% of the progeny produced by crosses between transgenic F1 and wild-types contained the AFP genes. These results demonstrate that stable germ-line transformed Atlantic salmon can be produced.