1. Chromosome number and behaviour are described in the three British species of the family Tetrigidae (Orthoptera): Tetrix undulata (Swrb.), T. ceperoi (Bol.) and T. subulata (L.). In all three species the male number was found to be 2n=12+X, the number possessed by all other previously described representatives of the family. 2. A number of meiotic variants were observed, which included cases of non-staining segments, polyploidy and failure of pairing. 3. In two individuals, of T. ceperoi B chromosomes were found. One possessed one B, the other two. 4. Male meiosis is characterised by two unusual features: a) An extremely diffuse early diplotene is present, in which the individuality of the bivalents is not resolvable: only the condensed allosome and the centric heterochromatic blocks are visible in the otherwise faintly granular nucleus. This diffuse stage reflects great autosomal synthetic activity, as a result of which the diameter of the nucleus is more than doubled. b) Chiasma formation is usually strictly localised. One chiasma invariably forms in a very small terminal distal segment of each autosome: markedly subterminal chiasmata are never found. Ring bivalents were observed in only five cells: all present in the one individual possessing a single B chromosome. Chiasma localisation and its mechanism are discussed. 5. A revised list of members of the family Tetrigidae so far studied cytologically is presented, together with their present taxonomic status. 6. Interesting variation in allosomal size is present in the otherwise remarkably constant complements of the species so far described.