Mosquitoes are the most important arthropod disease vectors, transmitting a broad range of pathogens that cause diseases such as malaria, lymphatic filariasis, and yellow fever. Mosquitoes and other insects are able to mount powerful cellular and humoral immune responses against invading pathogens. To date, most studies have concentrated on the humoral response. In the current study we describe the hemocytes (blood cells) of the yellow fever mosquito, Aedes aegypti, by means of morphology, lectin binding, and enzyme activity and immunocytochemistry. Our light and electron microscopic studies suggest the presence of four distinct hemocyte types: granulocytes, oenocytoids, adipohemocytes, and thrombocytoids. We believe granulocytes and oenocytoids are true circulating hemocytes, but adipohemocytes and thrombocytoids are likely adhered to fixed tissues. Granulocytes, the most abundant cell type, have acid phosphatase and alpha-naphthyl acetate esterase activity, and bind the exogenous lectins WGA, HPA, and GNL. Phenoloxidase, an essential enzyme in the melanotic encapsulation immune response, was detected inside oenocytoids. This is, to our knowledge, the first report that has detected phenoloxidase inside mosquito hemocytes at the ultrastructural level. These results have begun to form a knowledge base for our ongoing studies on the function of Ae. aegypti hemocytes, and their involvement in controlling infections.