Sexual size dimorphism was quantified for a population of the diving beetle Laccophilus maculosus Say in Northern Ontario, Canada. Consistent with other populations of this species, but in contrast to the general pattern in insects, males were significantly larger than females in terms of body length, elytral length, and pronotal width. There was a significant multivariate difference in body size between the sexes, and discriminant function analysis identified 68.87% of individuals as the correct sex based on measurements of body size. Males also had significantly longer femora, longer tibiae, and larger tarsi on the pro- and mesothoracic legs.