In the course of benthic sampling during 1981 and 1982 in Lake St. Louis, Que., 28 water mite larvae (Hygrobatidae: Hygrobates) were found parasitizing caddisfly larvae (Leptoceridae: Nectopsyche, Oecetis, Ceraclea). Two other water mite larvae (Unionicolidae: Unionicola) also were found: one on Nectopsyche from Lake St. Louis, and the other on Limnephilus (Limnephilidae) from the Magundy Stream, N.B. Prevalence of parasitism ranged from 8 to 42%. Mite distribution among available hosts suggests host selection by the mites or the absence of mites from certain microhabitats. Parasitism of fully submerged insect larvae by mite larvae represents a further derivation from ancestral behavior patterns. This paper describes five of the seven documented cases of such parasitism on Trichoptera larvae.