The larvae of leaf-mining insects are difficult to rear in the laboratory and large-scale insect surveys must often rely on the identification of immature stages for their information. The need for a larval key to the birch leaf-mining saw-flies in Ontario became apparent when a complex of three species, Profenusa alumna (MacG.), Fenusa pusilla (Lep.), and Heterarthrus nemoratus (Fall.), was first discovered in 1955 (Lindquist 1955). Larvae of the three mentioned species have been described in recent years by Watson (1959), Friend (1933), and Peirson and Brower (1936), respectively. Additional descriptions of the larvae of F. pusilla and H. nemoratus were made by Daviault (1937). Information on distribution and seasonal occurrence was obtained from collections made by forest biology rangers over a 3-year period. Brief notes on the biology of the species, for comparison purposes, follows the larval key.