A collection of thambematids (Crustacea, Isopoda, Asellota) from depths of 1400–2900 m in the Rockall Trough, west of the British Isles, is considered. The type species of the genusThambemaStebbing,T. amicorumStebbing, is rediagnosed and three new species,T. golanachum, T. tanum, andT. fiatum, are described. The generaThambemaandMicrothambemaBirstein are contrasted and taxonomic characters separating these genera are provided. The difficulty of providing effective definitions for higher category taxa is discussed and a new definition is provided for the Asellot