The effect of cycloheximide on cell organelles was investigated at the level of ultrastructure, with an emphasis on the ribosomal behaviors. The antibiotic, at low concentrations, primarily caused an orderly alignment of ribosomes on endoplasmic reticuli (ER), giving rise to rough-surfaced ER. Development of membrane systems was also a characteristic feature of the cells treated at low concentration or incubated for short period of time. Prolonged incubation or treatment at higher concentrations led eventuaily to the degeneration of cytoplasmic and nuclar organizations, the aggregation of ribosomal debris being one of the conspicuous features. On the contrary, no major structural change was observed of the chloroplasts and mitochondria which are both resistant to the inhibitory action of cycloheximide. These results were discussed in relation to the inhibition of protein synthesis by the cycloheximide.