ALD muscle development was studied from day 2 to week 15 in males of two turkey strains. At 15 weeks, the heavy-weight (HW) strain weighed 2.2 times as much as the light-weight strain (LW). Morphometric and immunocytochemical analysis showed the presence of small fibers in KW ALD muscle which simultaneously accumulated ventricular and embryonic fast myosin heavy chain isoforms. The appearance of these nascent myofibers suggests that hyperplasia contributes to the growth of HW ALD muscle.