In the course of screening for the substances suppressing bleb formation of K562 cell induced by phorbol 12, 13-dibutyrate (PDBu), an inhibitor, chaetoglobosin A (CgA) was isolated from a cultured broth of unidentified fungus. CgA showed a strong inhibitory activity with the $IC_{50}$ value of 60 pM against bleb formation on K562 cells induced by PDBu, but it did not inhibit the activity of protein kinase C (PKC) in vitro. The inhibitory activity of CgA might be due to the modulation of actin filaments on the cell membrane. CgA exhibited strong cytotoxicity against various human cancer cell lines.