The ultrastructure and division of the pyrenoids in a green colonial alga Coelastrum sp. was investigated. The pyrenoidal matrix consists of three components: helical fibrils, dense granular particles measuring 50-60Å in diameter and interspersed electron transparent areas. There are no tubules or membranes within the pyrenoid matrix. The present investigation confirms the division of the pyrenoids in this species. The pyrenoid is divided into two portions by apparent intrusive growth of the starch plates around the pyrenoid girdle. A circular furrow develops on the surface of the starch plates, expands centripetally, and cuts the pyrenoid into two.