In a selection experiment, halothane gene-free Swedish Yorkshire pigs were performance tested (25–90 kg) for improved lean tissue growth rate (LTGR) on two diets: high protein (HP 18.5 CP), and low protein (LP 13.1 CP). LTGR was estimated from ultrasonic measurements of fat and muscle depth and repeated weighings. 2885 pigs were tested in generation zero and four selected generations. The selection intensities were 17% for males and 54% for females. Genetic parameters were analysed with restricted maximum likelihood (REML) using a reduced animal model. Total response (phenotypic trend) was in HP +43 and in LP +25 g/day, and the genetic trend was HP +33 and LP +30 g/day. The heritability for LTGR was 0.37 in both lines. Reasons for the different rates of progress in the two lines are discussed.