This thesis is an amalgamation of and a contribution to the knowledge of the Ephemeroptera of British Columbia, an order which has been largely neglected in Canada, and particularly in this province. The three families Heptageniidae, Ephemeridae, and Baetidae are replaced by the five superfamilies Heptagenioidea, Leptophlebioidea, Caenoidea, Ephemeroidea, and Prosopistomatoidea proposed by Edmunds and Traver (1954). 146 species in 23 genera have been recorded in British Columbia. Of these, 62 as yet unidentified species are designated by number. A checklist of all recorded species, and keys and descriptions of taxonomically important criteria of the species and higher ranks are included herein. A preliminary investigation of the ecology of the nymphs collected by the author in the Alouette River and by Dr. G.G.E. Scudder in the lakes of the Chilcotin, Cariboo, and Nicola Valley regions revealed no "preference" of the nymphs for the middle or edges of the river or for particle size of the substrate. An accurate determination of the latter, however, is not obtainable with the type of collecting equipment employed during this study. Some "preference" was shown by a number of species for specific rates of current flow. No correlation between the species of nymphs and the pH or the salinity of the environment was determined. Suggestions are put forward for the extension of our knowledge of the taxonomy, ecology, and biology of the Ephemeroptera. / Science, Faculty of / Zoology, Department of / Graduate