Abstract.  An unknown beetle collected in the Territorio Federal Amazonas, Venezuela, and designated as belonging to a new family, is described (as Meru phyllisae gen.n. , sp.n. ) and illustrated by line drawings and scanning electron micrographs. The new family, Meruidae, is diagnosed and assigned to the suborder Adephaga with a discussion of some characters and likely phylogenetic affinities. With a body length of 0.85–0.9 mm, M. phyllisae is the smallest known member of aquatic Adephaga. Based on the observed skeletal and behavioural characters to date, the taxon appears to represent the sister clade to the remaining families of Dytiscoidea, but also has features suggesting a close relationship among Noteridae and/or Haliplidae. A description and photographs of the habitat are included, with a list of other associated water beetle taxa. Some observations on the behaviour of captive beetles are given. The minute ‘comb‐clawed cascade beetles’ are known only from the type locality, El Tobogán, where streams flow among exposed areas of bedrock in the geologically ancient Guyana Shield region.