We examined the lengths, weights, condition factors, and hepatosomatic indices of juvenile rainbow trout (Oncorhynchus mykiss) from four full-sib families, each segregating at the temporal regulatory locus Pgm1-t, and the concentrations of RNA, DNA, and protein in their livers and white muscle. In three families, fish with phosphoglucomutase-1 (PGM1) activity in liver (Pgm1-t(b) fish) are significantly longer than their full-sibs lacking activity for liver PGM1 (Pgm1-t(a) fish). Hepatosomatic indices tend to be higher in the Pgm1-t(b) fish than in their Pgm1-t(a) siblings. RNA/DNA ratios in the liver of Pgm1-t(b) fish are significantly higher than those of Pgm1-t(a) fish in two families and marginal in a third. However, no significant differences were detected in a parallel analysis of nucleic acids and protein in white muscle, where PGM1 is expressed in all fish. In a separate experiment, Pgm1-t(b) fish were significantly heavier in all five families, had significantly higher condition factors in two families, and had marginally lower standardized oxygen consumption rates in three families.