Skeletal muscle growth in two genetic lines of pigs that differed in total muscle content was studied at live weights of 23, 45, 68, 91, 104, 118 kg. Total physically separable muscle and cross-sectional area of the longissimus dorsi muscle were greater in the muscular than in the obese genetic lines. Above 45 kg, animals in the muscular genetic line had less total separable fat than animals in the obese line, but the two lines did not differ in total physically separable fat at 23 and 45 kg live weight. Hence, these two genetic lines may differ in weight at which maturity is reached as much as in inherent propensity for obesity. Longissimus muscle form the muscular line had more water, less protein, and less lipid than longissimus from the obese line. DNA and RNA concentration, total DNA and RNA content, and RNA/DNA ratio of the pituitary and liver did not differ between the two genetic lines. Above 68 kg, longissimus from the muscular line had higher DNA and RNA concentrations than longissimus from the obese line; this difference did not exist between 23 and 68 kg. RNA/DNA ratio of the longissimus muscle was greater and protein-to-DNA and protein-to-RNA ratios in longissimus were lower in the muscular than in the obese line. Total DNA content of physically separable muscle increased 2.0 (obese) to 2.7 (muscular)-fold between 23 and 118 kg; hence, number of muscle nuclei increases during growth. Total DNA content of physically separable muscle was greater in the muscular than in the obese line and was the measurement most highly related to total muscle content.