Salps are free-swimming tunicates whose peculiar life history renders them ideal for developmental studies. The solitary salp reproduces asexually by budding a stolon containing the complete developmental sequence of the aggregate generation. The ultrastructure of developing locomotor muscle of the aggregate generation of Cyclosalpa affinis was studied. The early muscle contains essentially non-striated myofibrils. However, in transverse sections, , areas indicating early I-band A-bands can be recognized. As development continues, the number of fibrils increases, the Z-line appear, and the fibrils contain more recognizable striations. The fully developed muscle has the caracteristic structure of striated muscle. Longitudinal sections show sarcomeres with irregular and discontinuous (perforated) Z-lines; H-zones are not apparent. No M-lines are seen. Throughout development, the ratio of thin to thick myofilaments is always 2:1, the ratio found in all vertebrate striated muscle. Other finding in C affinis suggest that: (1) multinucleated muscle cells are formed by the fusion of mononucleated cells, (2) membranes of adjacement mononucleated cells destined to fuse form myelin figures, and (3) these myelin figures become closely associated with mitochondria.