Studies on population increase of the alfalfa leafcutter bee (Megachile rotundata Fabr.) were conducted in northern areas of Alberta and British Columbia (lat. 55°N. to 58°30′N.). Polyethylene shelters with one side open, placed over the domiciles, raised domicile daytime temperatures by 4°–10 °C, increased the duration of activity and the pollinating range of the bees, and resulted in an increased number of cocoons. Population increase was 2–5 times greater when hives and shelters faced south than when they faced east or west. Within a shelter bees also preferred hives with southern exposure. Movement of domiciles in mid-season for distances of 150 m or more caused a reduction in bee populations. Bees preferred wood to polystrene nesting material, old to new material, and surface stained material with patterns to unstained material. Severe mould development in polystrene nesting material affected development of larvae and emergence of pupae. Since population increase of M. rotundata, by utilization of polyethylene shelters and by proper directional orientation, is successful, re-establishment of a major alfalfa industry in northwestern Canada is now possible.