There is increasing evidence for complex dosage effects on gene expression, enzyme activity and phenotype resulting from induced ploidy change. In this study, ocean-type chinook salmon were bred using a 2 · 2 factorial mating design to create four families and test whether triploidization resulted in changes in growth performance and smolting. Eggs were pressure shocked after fertilization to create triploid fish from a subset of each family. In June, fish were sampled for size, plasma insulin-like growth factor 1 (IGF-1), gill Na + -K + -ATPase activity, and expression of two Na + -K + -ATPase a subunits in the gill. Diploids were significantly heavier than triploids, and there were significant differences due to family. Despite a significant positive correlation between plasma IGF-1 and fish size, plasma IGF-1 did not differ between diploid and triploid smolts. Diploids also had significantly greater gill Na + -K + -ATPase enzyme activities than triploids and there was a strong family effect. Gill Na + -K + -ATPase a1b isoform expression differed significantly by family, but not ploidy, and generally families with lower Na + -K + -ATPase enzyme activity had higher a1b isoform gene expression. Na + -K + -ATPase a1a isoform expression did not differ among any of the groups. Although diploids were larger and had higher specific activities of Na + -K + -ATPase in the gills, there was no difference in gene expression or circulating hormone levels. The strong family effect, however, suggests that strain selection may be useful in improving performance of triploids for aquaculture.