Second and third instar mosquito larvae of 34 Canadian species belonging to the genera Anopheles, Aedes, Culex, Culiseta, and Wyeomyia may be reliably and simply separated by the absence or presence of the thoracic setae, 8-M and 7-T, and even more so by the number of the metathoracic setal support plates. As the 7-T seta is always inserted on a plate, a larva in the third stage will bear one more plate than in the second stage. In the species studied, the number of metathoracic plates is constant for a given genus at a given instar. Thus an intermediate instar larva, once identified to genus, can be assigned to second or third instar fairly readily by counting the number of setal plates on one side of the metathorax.