Microscope observations of chains of Chaetoceros armatum T. West, collected from its natural habitat at Copalis Beach, Washington, have been made. A regular daily periodicity in the occurrence of cell division stages and length of the chains was found on each occasion (November, February, April may, August). During the night, short chains of 3 cells predominated. In early morning hours, average chain lengths increased due to cell elongation, and nuclear and protoplast division occurred. This correlated with the time of day when cell masses of C. armatum first appeared at the surface of the water. In the late afternoon, the new silica values of the daughter protoplasts were formed. Formation of special bristle-bearing end values effected the separation of long chains into shorter chains composed predominantly of 3 cells. (Cultured material was used to verify some of these stages,) This correlated with time of day when cell masses disappeared from the water surface and dispersed. [ABSTRACT FROM AUTHOR]