A diallel cross with four strains of spring-spawning rainbow trout ( Salmo gairdneri) was carried out in two replicates to estimate average direct genetic effects, maternal genetic effects, general combining ability, heterosis, and reciprocal effects on growth and carcass performance. All matings within each replicate were performed the same day. Separate hatching and rearing were used for different genetic groups up to a size of 45 g, with common testing after freeze-branding up to a size of 220 g in two production environments (pond and silo). The average hatching rate of all mating groups was as high as 68.9%, the hatching rate of the crossbreds being slightly higher (+2.5%). For some reciprocal crosses, hatching rate was 15% higher than the midparent rate. During the rearing and fattening period the purebred strains showed significant differences in growth performance, with average line effects being more important than maternal effects. Differences in the general combining ability were also significant. For some strains, this was primarily caused by high average line effects while others showed a high general line heterosis. The mean growth performance of all crossbreds was not significantly different from that of the purebreds. Significant line differences were found for the percentage of fillet, head and fins, and offal, and for the percentage of fat and dry matter in the carcass. The crossbreds had a significantly higher percentage of fillet than the purebreds. However, ranking of the genetic groups according to their absolute weight of fillet was not different from a ranking by total weight. Significantly different average line effects observed for all traits indicated that a distinct strain differentiation existed. Consequently, selection of superior strains with regard to specific traits is of considerable importance.