1. As wild vertebrates are increasingly studied in more than one area or at more than one time instraspecific variation in social systems (IVSS) is increasingly observed. Intraspecific variation has been observed in a number of social systems. Individuals within a single species may be alone or in groups. Their spacing system may be territoriality, coloniality, lekking, dominance or despotism. They may breed monogamously, polyandrously, polygynously or promiscuously. Either both parents or a single parent may care for the young. The parents may or may not have helpers. Parents may or may not pool their young and care for them communally. 2. The evidence so far suggests that most species manifest one of two alternative systems of the same general type, i.e. one of two mating systems or parental care systems. This suggests that there are constraints on their flexibility. These constraints probably include the psychological complexity of the shift from one alternative to another, the compatibility of a particular system with other aspects of the species' natural history, and the species' phylogenetic history. 3. Several ecological variables have so far been identified as correlates of alternative social systems within a species. These variables are generally those currently considered important in interspecific socioecological analysis: differences in 1) the distribution and abundance of food, 2) the level of competition for food, 3) the level and kind of predator pressure, 4) the level of population density and 5) habitat saturation. 4. Alternative social systems are usually viewed as adaptive responses to the costs and benefits of the observed social system compared to alternative social systems. In this respect, IVSS is treated as intraspecific socioecology. 5. Intraspecific socioecology differs profoundly from interspecific socioecology in the approach taken in accounting for observed social systems. Species differences are typically assumed to be caused by genetic differences resulting from distinct natural selection histories in the populations under consideration. IVSS, on the other hand, often involves facultative switching between alternatives that are clearly not traceable to genetic differences. IVSS may be produced by a number of proximate mechanisms. Nutritional state, hormone levels and particular experiences can all predispose animals to different degrees of aggressivity, which in turn may influence the form of social system that emerges from their interactions. Different experiential histories may make one animal inclined to be a member of a group, and another inclined to be solitary. A high level of one kind of social activity may preclude engaging in another. For example, a territorial male under high intruder pressure may be too busy defending space to give paternal care. 6. Progress in the description and interpretation of IVSS will enrich animal behavior as a discipline. In addition, the ability to predict, and possibly even influence, rapid changes in social systems may contribute substantially to conservation strategies.