Goniodomin A, a polyether macrolide isolated from the dinoflagellate Goniodoma pseudogoniaulax, caused morphological change in 1321N1 human astrocytoma cells. Observation by scanning electron microscopy revealed that the shape of the cell became refractile from flat in response to goniodomin A. This morphological change was accompanied by the outgrowth of a needle-like structure from the cell surface. Goniodomin A increased the intracellular content of filamentous actin in a concentration- and time-dependent manner. It also caused the change in filamentous actin distribution in the cells. The elongation of filamentous actin was observed in goniodomin A-treated cells. These results suggest that goniodomin A induces morphological change by increasing the content of filamentous actin in non-muscle cells.