Comparative ultrastructural investigations of the effects of 0.005mcolchicine on the organization of cellular components inChlamydomonas eugametoswere made at intervals during and after treatment. Periodic observation of colchicine‐treated cells revealed curling of the Golgi cisternae and increased vesicle production by the Golgi apparatus, followed by a continuous increase in number of that organelle. Elongated mitochondria were observed to bo an early, but transitory, response, as was a marked proliferation of the nuclear envelope and endoplasmic reticulum; the latter was followed by an increase in nuclear size and convolution. Colchicine‐treated cells also manifested stratification of cell walls and replication of many cellular components. Nuclear anomalies, cytoplasmic compartmentalization, and unusual associations of several organelles were also observed. Initial manifestations of recovery were reorganization of the stigma granules and alignment of cytoplasmic vacuoles. Electron microscopy ofapparentlyrecovered cells indicated their organization to be similar to that of the controls.