The algae are important constituents of the living, changing complex that we call the soil. Soil contains not only minute mineral fragments but also water, gases, dead plant materials, insect larvae, nematodes, protozoa, bacteria, fungi and algae. This is an intricate association and the algae have a part in it. They add organilc matter to the soil, influence chemical changes therein, are known to fix nitrogen and are a food source for small animal life in the soil. Several workers throughout the world have listed the various algaethey have found in the soil (Bristol-Roach, 1927; Fritsch, 1922; Fritsch and Salisbury, 1915; Russell, 1923) and a few have studied the activities of these micro-organisms (Bristol, 1920; Petersen, 1935; Skinner, 1932). Most of these studies were made in Europe. In this country as early as 1912, W. W. Robbins examined 22 soils and found 12 species of algae living in them. Later, G. T. Moore and associates (1919; 1926) listed 31 species of algae found existing in soils at the Missouri Botanical Gardens, some of them rather deep below the surface. In 1925, Bonar and Goldsmith made a study of desert soils in the Southwest and found that such soils contained a total of 14 genera of algae; no specific list was published. Lowe and Moyse (1934) listed 27 species of algae from soils of Manitoba, Canada. The limited information on soil algae in this country suggests a need for further inventory, especially in the intermountain West. It is the purpose of the following paper to list such algae as have been found in Colorado soils, with brief mention of the conditions of their habitat.