Presented is an illustrated flow chart intended to facilitate identification of some common genera and species of Chironomid larvae of North America. Some brief remarks on the morphology and physiology of the family are given. Some of these species are actively favored by pollution and their presence, especially in the absence of intolerant forms may be taken as a strong indication of the presence of organic pollution. Other Chironomids conversely are highly intolerant of pollution, and a diagnosis of 'pollution' based solely on their presence as 'Chironomid larvae', would be a gross error. The importance of generic species identification in interpreting the pollutional significance of this group is thus self-evident.