The Osoyoos Lake problem is one of excessive algal populations creating nuisance conditions for recreation and agriculture. During the summer of 19 68 (May 28 to October 14) an extensive water sampling program was carried out to determine phytoplankton communities present and some of the physical and chemical factors influencing their growth. The lake was monitored using a system of three transects at predetermined sections across the lake and four fixed sampling locations on each transect. The sites on each transect were sampled at intervals of approximately two weeks. Mud samples were collected at each site once a month. Samples were also collected on a two week basis from various waters which eventually enter Osoyoos Lake. The purpose of this was to gain some indication of nutrients contributed to the lake by inflowing waters. Osoyoos Lake gave rise to a major blue-green alga bloom which persisted throughout June and July. This Anabaena flos-aquae bloom was followed by smaller populations of Fragilaria crotonensis, Dinobryon sertularia, a late summer pulse of A. flos-aquae, and at the southern end of the lake considerably large populations of Melosira italiea and Oscillatoria acutissima. A discussion of the geological, physical, chemical and morphological factors possibly combining to create such growths in the lake is presented. It was generally concluded that high levels of nitrogen, phosphorus, calcium and dissolved solids were favouring the enhancement of eutrophication in Osoyoos Lake. Climate, lake morphology and the edaphic factor were also favourable to productivity. There is evidence to support the statement that sewage effluent and industrial wastes are sources of phosphorus and nitrogen build-up in the lake. In addition to these sources, sewage seepage, agricultural drainage and nitrogen fixation are believed to be contributing to the accumulation of lake nitrogen. The continuous application of water to the fertilized orchards surrounding the basin is thought to be instrumental in leaching fertilizers and minerals natural to the land into the lake at an accelerated pace. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate