Transgenic rainbow trout (RT) were generated in earlier experiments by an in vivo modification of the chromosome-mediated gene transfer technique. This involved fertilizing albino RT eggs with gamma-irradiated brook (speckled) trout (ST) sperm and then heat shocking the eggs to induce second polar body retention. Striped pigment patterns were stable in six mosaic individuals between the ages of 7 and 14 months. We estimated the total ST DNA contribution in adult transgenic fish using DNA dot blot analyses. An average of 7% ST DNA was obtained in analyses of 11 transgenic individuals. Nine of 55 transgenic individuals examined expressed a ST-specific isozyme in adulthood; some were mosaic for foreign gene expression. Variable numbers of autonomous chromosome fragments were maintained in 10 adult transgenic individuals examined. Five transgenic RT were backcrossed to albino RT. Chromosome fragments were inherited by 29 of 31 backcross embryos examined. Additional active nucleolar organizer regions (NORs) were present on some fragments and may also have been integrated into some host chromosomes. This method may have numerous applications for basic and applied genetic research in fish.