Scanning electron microscopic studies of myoblasts from 11- to 13-day-old chick embroyonic breast muscle cultured on collagen-coated glass coverslips showed six stages of development into multinucleated myotubes: (1) growth of flattened, spread-out cells for 20-30 hr following initiation of monolayer cultures; (2) extension of microprocesses (1-150 microM) from cells that have become spindle shaped; (3) contact and adherence of microprocesses from adjacent cells; (4) thickening of fused processes; (5) approximation of the cells; and (6) coalescence of the cells to form a spindle-shaped myotube. When the calcium-ion concentration in the growth medium was lowered--either by increasing the concentration of ethylene-glycol-bis(aminoethyl ether)N,N'-tetraacetate (EGTA)or by decreasing the cconcentration of free calciumion used--the number of microprocesses present on the cells was reduced. Presumably, however, these microprocesses could still fuse together, provided that the calciumion concentration was greater than 160 microM. Indirect immunofluorescence assay with actin-specific antibody indicated that actin is a major component of the myoblasts' microprocesses. Cytochalasin B (5 microgram/ml) caused the microprocesses to retract within 15 min and the myoblasts to round up and detach from the glass substrate. This was presumably caused by the action of the drug on actin filaments.