Summary. Two different techniques have been adapted for Micras-terias denticulata to depict the actin cytoskeleton of both untreated and inhibitor-treated developing cells: the "quickstaining method", where the cells are fixed in a mixture of glutaraldehyde and formaldehyde followed by staining with phalloidin without embedding, and the "methacrylate method", where the cells are also fixed by aldehydes and where the embedding medium is removed prior to incubation with an actin antibody. Both methods produce sufficient preservation and visualization of actin microfilaments (MFs) and confirm earlier observations on the presence of a cortical actin MF network in both the growing and the nongrowing semicell as well as of a basketlike MF arrangement around the migrating nucleus. The results show that a network of actin MFs is essential for the proper development of the young lobes of M. denticulata. Early developmental stages expanding uniformly at the beginning of growth lack any netlike actin MF arrangement. The actin cytoskeleton in developing cells treated with the actin-targeting agents cytocbalasin D and latrunculin B is markedly influenced. Cytochalasin D, which produces the most pronounced effects, causes a breakdown of the network of actin MFs, resulting in bright actin clusters as well as in short and abnormally thick actin fragments particularly in cortical cell regions. In latrunculin B-treated cells remnants of the former actin MF network are still visible, yet most of the actin cytoskeleton appears collapsed and is reduced to short filament pieces. The disturbance of the actin MF system visualized in the present study correlates with the severe morphological and ultrastructural changes occurring in desmid cells as a consequence of both drugs. The dinitroanilin herbicide oryzalin, known to deploymerize cytoplasmic microtubules, causes also an impairment of the actin cytoskeleton in M. denticulata though not sufficient to influence normal cell growth and differentiation.