Pekin and Muscovy breeds of domestic ducks were reared from hatching to 10 weeks. Males and females of each breed were killed at weekly intervals and sartorius muscles were removed after the onset of rigor mortis. Fiber diameters were measured from macerated preparations after fixation with formaldehyde. All volumetric data were corrected to a common sarcomere length. Longitudinal and radial growth of muscles reached their maximum velocity during the fourth and fifth weeks, respectively. In Muscovies, heavier muscles in males relative to females were due to increased muscle length and cross sectional area. Faster early growth of Pekins relative to Muscovies was due to muscle length and fiber diameter. Histochemical analysis of supracoracoideus muscles frozen immediately postmortem showed that both adenosine triphosphatase and succinate dehydrogenase activity were negatively correlated with muscle fiber cross sectional area.