1. The hemocytes of Rhodnius prolixus have been studied with phase contrast microscopy, after supravital staining, and in fixed and stained smears.2. With phase contrast microscopy, the following categories of circulating cells can be readily identified: (a) non-dividing and mitotically-dividing prohemocytes, (b) non-vacuolated and vacuolated plasmatocytes, (c) intact and quickly lysing granular hemocytes, (d) oenocytoids with and without special cytoplasmic inclusions, (e) adipohemocytes, and fat body cells, and (f) granulocytophagous cells.3. This classification and terminology are compared with those of Wigglesworth. It is suggested that the cell which Wigglesworth terms an oenocytoid is more comparable to the granulated blood cells of other insects and may be referred to as a granular hemocyte. It is suggested that the cells which Wigglesworth refers to as large non-granular spindle cells and non-phagocytic giant hemocytes are comparable to the oenocytoids of other insects.4. Vacuolation of plasmatocy...