Mesnilium is a genus of parasitic alveolates belonging to the phylum Apicomplexa. Its vertebrate hosts are fish. The vectors are not presently known but are thought likely to be leeches.
Merogony occurs in erythrocytes and reticulo-endothelial cells. Gamogony occurs only in erythrocytes and pigment granules are present only in microgamonts and macrogametes.