Three-dimensional reconstructions based on serial sections through cells of the chloromonadophycean alga Gonyostomum semen Diesing indicate that there are many small unbranched mitochondria in log-phase cells. These organelles are approximately ellipsoid in shape with their longest axis ranging from 0-7 to 2-0 micrometer in length. Occasionally mitochondria reaching up to 3-6 micrometer in length have been observed: these have the same general morphology as their smaller counterparts. Preliminary observations indicate that cells of a related alga, Vacuolaria virescens Cienkowsky, also possess many small unbranched mitochondria. These observations are discussed in relation to recent reports that some cells contain a single branched mitochondrion or, at most, a small number of these organelles. It is suggested that mitochondrial number may be related to cell volume and that large cells may contain many mitochondria.