The life cycle of a Haplosporidium found in the aquatic snails of the Mackinac Straits region of Michigan has been studied. This represents the first report of this genus in fresh-water snails or indeed fresh-water invertebrates. It is the first record of these parasites from North America. The Haplosporidium develops in the digestive gland of the host snail. Many of them are observed to be cytozoic located in the part of the mucosal cells that is distal to the lumen of the gland while others seem to lie in the submucosal region of the tissue. The truncate spores are operculated with dimensions of 4.25 micron x 3.8 micron recorded from one host. The earliest stage of the infection in the tissues is recognized as small amoebulae that grow into plasmodia that divide into two distinct types of small forms: A) small irregularly shaped amoebulae with 1-4 nuclei that reinfect other cells for another cycle of schizogony and B) Binucleated individuals that undergo sporogony. The B form is found in spherical or oval cysts with mean diameters of 43.3 micron which contain from 78-221 spores. The Haplosporidium has been recovered from the following genera of snails: Stagnicola, Physa, and Heliosoma. Of 68 Stagnicola brought into the laboratory October 1958, 50% died during the first week. Of this number 90%, 87% and 50% of those dead on days 1, 3 and 7 respectively were infected. None of the remaining 30 snails died until the termination of the experiment on the 28th day. Only 10% of these were infected.