Insect Sf9 cells usually elaborate a highly characteristic single process when infected with a baculovirus encoding recombinant human tau. The processes are unbranched, of uniform caliber, and contain bundles of microtubules. Because taxol treatment alone does not induce process outgrowth in these cells, it is believed that tau confers properties on microtubules that permits the conversion of microtubule assembly into the formation of processes. Here we have analyzed the reorganization of both actin filaments and microtubules during process initiation. A zone of organelle exclusion representing the focal reorganization of actin at one pole of the cell anticipated process emergence. A relationship between actin organization and process emergence was also suggested by a shift from single to multiple process formation after treatment with cytochalasin D. The rate of process elongation doubled after cytochalasin treatment of tau-expressing cells. The increase in rate was due to the inhibition of the growth arrest phases which occur in the absence of cytochalasin. In contrast, Sf9 cells treated with cytochalasin after more than 20 h of tau expression were relatively resistant to the drug's effects. We conclude that actin and microtubules are specifically reorganized during tau-induced process outgrowth and that a dynamic relationship between actin filaments and microtubules effects process formation.