Polish Merino lambs (ewes and rams, n=322) and crossbred Polish Merino, Booroola and Suffolk lambs were raised for 8 weeks, then intensively fattened (rams, n=66) to a body weight of 35 kg. Lambs with a genotype of 75% Merino and 25% Booroola (MBM) had lower daily weight gains (181 g; P≤0.01) during raising than Merino lambs (MM, 212 g). This relationship was not found in lambs with a genotype of 25% Merino, 25% Booroola and 50% Suffolk (MBS, daily gain 215 g). Body weight gains (372 g) of MBS rams were larger (P≤0.01) during fattening than those of MBM (301 g) and MM (309 g) rams. MBS rams utilized less (P≤0.01) concentrate dry matter, crude protein and metabolizable energy (2.9 kg, 608 g and 35.29 MJ) per kg body weight gain than MBM (3.5 kg, 735 g and 42.63 MJ) and MM (3.5 kg, 721 g and 41.86 MJ) rams, respectively. The slaughter value of MBS and MM rams was similar, while that of MBM rams was inferior due to the higher fat content of the carcasses.