AbstractCalyculin‐A, an inhibitor of type 1 and 2A phosphatases, was applied extracel‐lularly to 3T3 fibroblasts. At 0.1 μM, calyculin‐A caused a marked increase in protein phosphorylation in both the cytosolic and insoluble cellular fractions. This effect was independent of external Ca2+. An immunoprecipitate, formed with an antibody to myosin, contained several cytoskeletal components. Increased phos‐phorylation following treatment with calyculin‐A was observed in vimentin, the 20‐kD myosin light chain, and an unidentified 440‐kD component. An enhanced level of vimentin phosphorylation was found in intermediate filament preparations from treated cells.Calyculin‐A also caused marked shape changes of 3T3 cells. Within minutes after addition of calyculin‐A (0.1 μM) cells became rounded and lost attachment to the substratum. Stress fibers, intermediate filaments, and microtubules, prominent in the attached control cells, were not evident in the rounded cells. Shape changes were reversible and after removal of calyculin‐A the rounded cells attached to the substratum, resumed a flattened shape, and were active mitotically. In the cells treated with calyculin‐A an unusual “ball‐like” structure was observed with transmission electron microscopy. This unique structure was 2–3 μM in diameter and was located close to the nucleus.The use of calyculin‐A adds further support to the idea that cell shape is controlled, at least in part, by concerted acti