Some aspects of thecal morphology in Ceratium hirundinella are redescribed by means of scanning electron microscopy. The internal anatomy of the cell has been clearly revealed for the first time in serial sections examined by transmission electron microscopy. An unusual sulcus, termed the sulcul aperture, has been found to open into a large ventral chamber which extends toward the center of the cell. The 2 flagella are inserted at one side of the ventral chamber. The ultrastructure of the flagellar canals and root systems arc described in detail. A microtubular strand has been found to run from near the nucleus to near the flagellar bases but not to conned with either. The fine structure of the theca, trichocysts, nucleus, and chloroplasts is basically similar to that of other dinoflagellates. Food vacuoles are reported. These contain material thought to be undergoing digestion. It is suggested that the food particles enter the cell by way of the ventral chamber, which is lined only by membranes, unlike the rest of the cell, which is covered by thick plates. The new findings are discussed in relation to the earlier work on the species and to the fine structure of oilier dinoflagellates.