IN the course of work on the experimental delimitation of botanical groups, the need has arisen for a term which can be applied to any specified assemblage of taxonomically closely related individuals. Such phrases as ‘local intrabreeding populations’ or ‘populations occupying a specific ecological habitat’ are cumbersome, and it is felt that a more concise terminology would be useful and, further, would focus attention on certain concepts undoubtedly of great importance in the study of intra-group variation. We propose the term deme* (from the Greek ) for this purpose, with appropriate prefixes to denote particular kinds of demes. For example, in a taxonomic group consisting of a number of potentially interfertile individuals all the individuals do not have an equal chance of interbreeding in nature. The tendency is for individuals in close proximity to interbreed more frequently with each other than with individuals at a distance, and thus small, more or less isolated intrabreeding colonies are set up. The distinctive features so commonly exhibited by local communities, for example, of sea plantain, provide evidence of this. These ‘breeding communities’ are likely to become increasingly important in the intensive study of evolutionary problems and we propose to name them gamodemes. It is clear that there are various degrees of isolation between such gamodemes, and when more experimental work has been done on this point it should be possible to devise a method of expressing degree of isolation quantitatively. At present, however, the concepts must remain somewhat vague, as is indicated in the definition given below.