Triploid fish hybrids frequently survive better than diploid hybrids. Tiger trout are a sterile hybrid between female brown trout Salmo trutta and male brook trout Salvelinus fontinalis; they have a poor early survival rate but a good growth rate. We produced triploid tiger trout by heat shock treatments of fertilized eggs and examined their survival and growth under hatchery conditions. Triploid tiger trout survived better to the initiation of feeding (34%) than did diploid tiger trout (5%) but had lower survival than brown trout (70%). Triploid tiger trout had similar survival to and better growth than brown trout when the two groups were reared together for 239 d after the initiation of feeding. Analysis of the asymmetry of bilateral meristic traits indicated higher developmental stability in the triploid hybrids than in the diploid hybrids. Expression of both parental alleles of enzyme loci, determined by protein electrophoresis, confirmed the hybrid nature of the offspring. Triploid hybrids could be distinguished reliably from diploid hybrids by differences in relative expression of maternal and paternal alleles at some, but not all, loci. Triploid tiger trout appear to be a vigorous hybrid that could be of particular value in situations where allsterile trout populations are desired for stocking.