Acrochaetium is a genus of very small marine red algae with a heterotrichous habit. Individuals are found on or in plants and animals in the lower littoral and sublittoral regions. The genus includes about 250 species. Many of the species descriptions are incomplete and inadequate, since they are based on minute morphological differences only, whereas information concerning life histories is scarce. Recent investigations show that Acrochaetium taxa hitherto regarded as separate species may represent either phases of a single life history or even phases in the life history of red algae placed in other genera (reviewed by Dixon 1970; West 1968; Woelkerling 1970). A second factor which casts some doubt on existing classification is the lack of knowledge concerning modifications due to the environment.