Tree holes containing water suitable for mosquito larvae were examined throughout southwestern Ontario. Larvae of Aedes triseriatus (Say) and Ae. hendersoni Cockerell were collected from 165 and 6 tree holes respectively. Both Ae. triseriatus and Ae. hendersoni were widely distributed in southwestern Ontario with Ae. triseriatus more abundant than Ae. hendersoni. Larvae of Anopheles barberi Coq., Culex restuans Theob., and Orthopodomyia signifera (Coq.) occurred also in tree holes in Ontario. The majority of the tree holes were cavities between trunks of sugar maple trees (Acer saccharum Marsh) and were located less than a meter above the ground. The diameter of the tree holes ranged from 3.0 to 41 cm. and the pH values of the water varied from 4.0 to 8.3.