The extremely halophilic archaea (family Halobacteriaceae) are the dominant heterotrophic organisms in hypersaline environments in which salt concentrations exceed 250–300 g l−1. During the last decades our knowledge on the taxonomy, physiology and biochemistry of the Halobacterium group has greatly increased. However, our understanding of the ecology of the halophilic archaea lags far behind the progess made in the study of other aspects of their biology. A few hypersaline environments, such as the Dead Sea and solar salterns, have been studied more in depth, using techniques such as lipid analysis to obtain information on the types of organisms present and measurement of uptake of labeled substrates to quantify the dynamics of bacterial processes. The results of these studies, in combination with the information obtained from laboratory studies of representative isolates of the Halobacteriaceae, enable the beginning of an understanding of the functioning of the halophilic archaea in nature.