A number of temperature-sensitive cdc- mutants of Schizosaccharomyces pombe that are affected in septum formation were analyzed with respect to their ultrastructure and the composition of their cell wall polymers. One mutant strain, cdc 16-116, has a cell wall composition similar to the wild type (strain 972 h-). However two other mutants, cdc 4 and cdc 7, show a higher galactomannan content and a lower alpha-glucan content. In all the mutants tested, total glucose incorporation, protein, RNA and DNA synthesis increased similarly to wild type over 3 1/2 h. After 2-3 h of incubation at the non permissive temperature -35 degrees C-, cell numbers remained constant although, increases in optical densities at 600 nm were observed. According to scanning electron microscopy, the mutants had aberrant shapes after 5 h of incubation at 35 degrees C. Transmission electron microscopy showed that cdc 3 is unable to complete septum formation. cdc 4 showed the most varied morphological shapes and aberrant depositions of cell wall material. cdc 8 exhibited a deranged plasma membrane and cell wall regions near of cell poles; an abnormal septum and several nuclei. cdc 7 showed elongated cells with several nuclei and with an apparently normal cell wall completely lacking in septum and septal material. cdc 16 showed more than one septum per cell.