The effects of cyclochlorotine (CC), a secondary metabolite of Penicillium islandicum, on cardiomyocytes and fibroblasts were investigated by fluorescent and electron microscopy. The in vitro application of CC on myocytes induced disruption of myofibrils and large accumulations of actin, myosin, alpha-actinin, and vinculin at the cellular processes. This agent also induced the formation of islands of myosin and alpha-actinin aggregates. Actin filament bundles in fibroblasts were either unaffected or disrupted completely, suggesting the presence of two, physiocochemically different, populations of fibroblasts. CC damage was dose-dependently reversible. Further studies will be required to explore the feasibility of using CC for analyzing the properties of contractile and/or cytoskeletal proteins.