NOT SO long ago it was generally said that the black rat, Battus rattus,had disappeared from Britain, ousted in unequal contest with the brown new-comer, Battus norvegicus.But to most naturalists interested in the subject it was known that little centres existed in wide-scattered areas where a black rat could be found if it was wanted. Now Colin Matheson, in an interesting account of results derived from his own observations in Cardiff and from a questionnairesent to medical officers of health in “approved ports”, discloses a somewhat disturbing situation about the black rat, the carrier of the flea, which in its turn disseminates plague (J. Anim. Ecol.,8, 76; 1939). Stringent regulations are in force to ensure that ships are kept free of rats, and that while ships are in port it should be made extremely difficult for inboard rats to make a passage to the shore. The result has been a general decline in the rat population of ships visiting Britain, although a number averaging 6-8 seems to be the irreducible minimum on rat-infested ships.