Phagokinetic tracks were used to determine the current direction of migration in 3T3 cells. Comparing this direction with the orientation of actin or tubulin-containing cellular structures by indirect immunofluorescence, the following results were obtained. First, the main actin-containing bundles were located at the bottom and tail end of 3T3 cells and ran parallel to the current or preceding direction of migration. Second, the 3 micrometer long rod-like structure (primary cilium), which contains tubulin and which has been observed by other investigators in transmission electron microscopy (Barnes, 1961; Sorokin, 1962; Wheatley, 1969) and in indirect immunofluorescence (Osborn and Weber, 1976), was oriented predominantly parallel to the substrate and to the current movement direction. It seems possible that the primary cilium has a role in the directional control of a migrating 3T3 cell, and that the main actin containing bundles act as substrate-attached rails along which the nucleus and bulk cytoplasm slide during displacement of the cells.