SUMMARY For the isolation of blue-green algae from soil and fresh water, tempera- ture is a selective factor of major importance. In a nutritionally non-selective mineral medium, which at 25" supported growth of blue-green algae and of many eucaryotic algae, the development of eucaryotic algae was almost com- pletely suppressed by incubation at 35". Both the number and variety of blue-green algae recoverable at 35" were greater than those recoverable from parallel cultures incubated at 25". When a combined-nitrogen source was omitted from the enrichment medium, only blue-green algae developed at both 25" and 35" ; the microflora consisted exclusively of heterocyst-forming filamentous types, other groups of blue-green algae being eliminated. The extension of the techniques of microbiology to the study of blue-green algae has been slow, and relatively few representatives of the group have so far been obtained in pure culture (Koch, 1964). The isolation of these organisms from natural sources would be facilitated if cultures could be enriched in them by methods which effectively counter-selected eucaryotic algae. One such method was discovered by Beijerinck (1902), who showed that blue-green algae of the Nostoc type can be selected by using a mineral medium devoid of a combined-nitrogen source. Subsequent studies (Allen, 1952; Fogg, 1956) suggest that the ability to fix nitrogen is largely restricted to blue-green algae belonging to the Nostocaceae and is by no means universal in the members of this class. Consequently, Beijerinck's enrichment method, although completely effective in eliminating competition from eucaryotic algae, yields an extremely restricted fraction of the total blue-green algal microflora. Apart from the nitrogen-fixers, the blue-green algae seem to have nutritional requirements very similar to those of other algae, so that the possibility of devising other enrichment methods based on the principle of nutritional selection does not appear to be promising. Among algae, thermophily is virtually confined to blue-green algae; and cultures can be effectively enriched in the thermophilic members of the group at temperatures in the range from 60 to 75" (Peary & Castenholz, 1964). However, this physiological property is also a relatively rare one within the group. A physiological study of a few pure strains (Kratz & Myers, 1955) suggested that even mesophilic blue-green algae may have a temperature range significantly higher than that characteristic of most other algae: the temperature optima were 32.5" for Nostoc muscorum; 35" for Anabaena cylindrica; and 41" for Anacystis nidulans. In commenting on these findings, Fogg (1956) remarked: 'it should be noted that the