Here we present data on a study of the species composition and structure of borokotóh, a habitat associated with central Amazonian seasonally black-water flooded forests. Borokotóh appears never to have been specifically studied before, being mentioned only once or twice in the literature (and then not under this name). This is the first report on the phytosociology of this vegetation association. Borokotóh is, on average, only some 20 m wide, but very species rich (107 species in 60 genera from 34 families in one 0.25-ha plot). It consists of a series of rounded hummocks of fine-grained mud, each 6-12 m in length and 2-3 m in height, separated by narrow sloughs. Hummocks are seasonally inundated almost to their flat tops, which remained dry in the years of the survey. Species are strongly stratified within the borokotóh, with over 78% of identified species being recorded in just one of the three hummock sub-divisions: top, side or in between-hummock slough. This specificity, almost certainly linked to inundation duration and waterlogging tolerance, may explain the high species diversity recorded. The study is significant in demonstrating that undescribed habitats still exist in Amazonia, and that biodiversity there may be structured in ways that are highly localized and habitat-specific. There is clearly a need for more detailed work on borokotóh and similar vegetation associations where high biodiversity may be associated with very specific local features.