More than 95% of the mosquitoes in southern Alberta develop in waste irrigation water. Sixteen species of four genera in the subfamily Culicinae were identified, namely, Anopheles earlei, Culex tarsalis, Culiseta inornata, Culiseta a2askaensis, Aedes vexans, A . cinereus, A . dorsalis, A . campestris, A . spencerii, A.jlauwcens, A . m i~o m a c n l i s , A . riparius, -4. stictictu, A. i n t r h s . A. cnbaphylb, A. mdaximm; and two species in the subfamily Chnoborinae, nameIy, Chanbnvvs am~rimnus and C. Jlnrqlcans. Weekly irrdex of the ntiml,er of larvae and pupae as the average numl~er per dip multiplicrl by the area in square yards, and total weeklv trap catches of adults of the seven moqt common species were comp~~ted for each of three irrigation difiricts, representing distinctly different farming prartice.9. Adults were more numcrorts in a sheltered than in a n own site, the males noticeably more so than the females. Moquitocc were more abundant in the older than in the newer irrigated districts, and improper farming and watermanagement practices favored irlcrease in n~i~ttbesu, even in weI1-planned irrigation districts.'Manuscript received August 7,1959. Contribution from the Veterinary-Medical Entomology Section, Canada Agriculture Research Station, Lethbridge, Alberta; from a thesis submitted to the Faculty of Graduate Studies, University of Alberta, in partial fulfillment of the requirements for the M.Sc. degree, 1958.