After initial patterning, muscle in the trunk and fins of teleosts grows extensively. Here, we describe muscle growth in zebrafish, with emphasis on the pectoral fin musculature. In the trunk, slow muscle fibers differentiate first. In contrast, slow muscle does not appear in the pectoral fin until the beginning of the juvenile period. Mosaic hyperplasia contributes to trunk muscle growth, and new fibers are apparent within the muscle as early as 6 mm standard length. In the pectoral fin muscle, mosaic hyperplasia is not evident at any examined stage. Instead, the predominant mode of hyperplasia is stratified. In larval pectoral fin muscle new fibers appear subjacent to the skin, and this correlates with the expression of myogenic genes such as muscle regulatory factors and Pax7. Our results suggest that regulation of fiber type development and muscle growth may differ in the pectoral fin and trunk. Developmental Dynamics 237:307–315, 2008. © 2007 Wiley-Liss, Inc.