Jasplakinolide, a naturally occurring cyclodepsipeptide from the marine sponge Jaspis sp., known to induce actin polymerization and stabilization in vitro, markedly influences the morphogenetic process in the green alga Micrasterias when used in concentrations higher than 3 microM. Development of Micrasterias is inhibited or strongly retarded, malformations occur, and large vacuoles are formed. At the ultrastructural level, dense abnormal accumulations of filamentous structures have been found indicating actin filament polymerizing activities of the drug in situ. Moreover, displacement of organelles and aggregations of endoplasmic reticulum (ER) cisternae have been observed. Microtubule (MT) arrangement and MT-dependent processes remain undisturbed. Cells allowed to recover from jasplakinolide treatment continue their growth but show severe changes in the cell pattern and displacement of organelles, suggesting that even after removal of the drug, some basic features for the morphogenetic process remain altered. Jasplakinolide might be a useful tool for investigations on actin-dependent processes in the future.