Vertebrate skeletal muscles contain a family of myosin heavy chain (MyHC) proteins whose expression varies with clonal origin, developmental age, hormonal state, and innervation patterns. The number of MyHC genes and their regulation have been intensely studied in mammals and birds. However, Comparatively little is known about MyHC expression in other vertebrates. To understand better MyHC regulation in amphibians, I have examined the fiber type expression of two tadpole stage-specific MyHC transcripts in Xenopus laevis. cDNAs for these transcripts, called MyHC E3 and MyHC E19, were used to synthesize digoxigenin labeled antisense RNA probes. In situ hybridization of these probes revealed that MyHC E3 was expressed through the myotome but was most abundant in the core fibers of the axial muscles, which are larval type II. In contrast, E19 was expressed most abundantly in the small diameter peripheral fibers of the axial muscles, which are larval type I. Transcripts of both genes were detected exclusively in skeletal muscles neither heavy chain was expressed in cardiac muscle, smooth muscle, or lymph heart muscle. E3 expression was first detected at late gastrula in both segmented and unsegmented dorsal mesoderm. It remained abundant throughout premetamorphosis in myotomal muscle, and was also found in the levator hyoideus and mandibularis muscle of the head, and in abdominal wall muscles. In contrast, E19 was first detected at tailbud in the tail tip and its expression spread anteriorly among larval type I and type II fibers of the myotome and head muscles during the next few hours of development. The patterns of expression suggest that MyHC E3 is expressed predominately in larval type II fibers, and MyHC E19 is initially expressed predominately in larval type I fibers but as development proceeds is expressed in both type I and type II.