Distribution of microtubules and other cytoskeletal filaments in growing skeletal muscle cells (myotubes) was studied in vitro by fluorescence microscopy using fluorescin-labeled antibodies and phalloidin, a specific antiactin drug. In the distal elongating tips of myotubes, microtubules were the major cytoskeletal elements; actin and intermediate filaments were much less abundant. On the other hand, colcemid- and nocodozole-treatments caused disruption of microtubles and also prompt retraction of growth tips to form myosacs, a type of deformed myotube. Actin filaments remained unaffected during the retraction. The difference in the distribution of the 3 cytoskeletal filaments in the region of growth tips was most remarkable in the case of those myotubes in the process of recovery from myosacs. In an early phase of recovery, the cellular processes extending from myosacs were enriched with both microtubules and intermediate filaments, but not with actin filaments. Later, when the processes became further developed, intermediate filaments were scarce at the extreme ends. Fluorescein-labeled actin introduced by a micro-injection method was minimally incorporated into filaments in the cellular processes. We conclude that microtubules make up the cytoskeletal element which is most responsible for elongation or spreading of growth tips of myotubes in vitro.