The factors governing the distribution of freshwater algae are doubtless quite similar to those important to many other plants, differing only in magnitude, availability and specificity of effects. The persistence of an alga in any environmental complex simply attests to the fact that there is more or less favorable interaction between the hereditary mechanism of the plant and the influence of numerous internal and external factors. It is well known that some algae grow and multiply in certain habitats; others merely exist in such places; and some never appear there. Moreover, numerous species of algae are widely distributed in many habitats while others are confined to special and sometimes quite local areas. Some species occur regularly in the same waters year after year, and others appear only occasionally or rarely. Such diversity of response to environment among the many thousand species of algae makes difficult the drawing up of generalizations regarding algal distribution. The Myxophyceae, or blue-green algae, completely lack sexual reproduction, and the hereditary mechanisms of these algae are practically unknown. They grow in habitats varying from aquatic to terrestrial, and from epiphytic to endophytic; they survive in ice and in hot springs having temperatures as high as 850 C; they occur in desert soil, tidal flats and many denuded soil areas; they flourish during the hot months of the year in shallow waters and often completely dominate the plankton of both fresh and salt water. These facts indicate that an explanation of the distribution of blue-green algae is not a simple one. It is important, however, that records be made of all data pertinent to their distribution. G. S. West (1916) was among the first to attempt a grouping of fresh-water algae on an ecological basis. He established subaerial communities, associated species on dripping rocks, and aquatic communities, each with certain limiting subdivisions. On the basis of periods of greatest abundance, duration of vegetative cycles and times of sexual reproduction, Transeau (1916) divided the algae of Illinois into seven ecological groups: winter annuals, spring annuals, summer annuals, autumn annuals, perennials, ephemerals (mostly plankton), and irregulars. There is an abundance of literature dealing with the composition, permanence and kinds of algal aggregations, and Tiffany (1951) has summarized the more important of these contributions, listing six more or less distinct ecological communities: hydrophytes, edaphophytes, aerophytes, cryophytes, endophytes and epizoophytes. The Myxophyceae have received considerable attention in such ecological studies as indicated above. Special note has been made in many papers on the abundance and often preponderance of species of blue-green algae as epiphytes in moist tropical areas; as carpets on moist ground, damp stones and wet woodwork; as attachments to dripping rocks in ravines, and to stones and boulders over which flows; as water blooms in lakes and ponds; and as living in