The interphase nucleus of the chloromonadophycean alga, Gonyostomum semen (Ehrenberg) Diesing, has a highly distinctive appearance. Interphase chromatin is readily distinguishable in both light and electron microscope preparations. It extends throughout the neucleus and frequently makes contact with the nucleoli and with the nuclear envelope. Among the chromatin filaments are large numbers of 35-46 nm diameter granules which occur singly or in clusters. The nucleoli are characteristically located in the posterior half of the nucleus and are composed of granular and non-granular components. Nuclear pores occur in slight depressions of the nuclear surface; their lumen has a diameter of approximately 75 nm and contains electron-dense material. The chromatin and the large numbers of nuclear granules are unusual and warrant further investigation.