An increasing number of organisms which are able to grow at high temperature has been isolated during the last few years.Our purpose in this paper is to analyze their properties from a microbiological point of view. The discussion will only deal with thermophilic procaryotes to the exclusion of blue-green algae (Castenholz, 1969) and actinomycetes (Cross, 1968) which have been reviewed recently. Organisms will be considered as thermophilic if they grow above 50°C, as extreme thermophilic or caldo-active if they grow at temperatures in excess of 70°C.