The Preliminary Check-list (Parke, 1953), with the three subsequent lists of additions and corrections (Parke, 1956, 1957, 1959) was intended to provide a fundamental basis for the preparation of a Flora of the British marine algae sponsored by the British Phycological Society. Changes, both taxonomic and systematic, have been so numerous in recent years that a new Check-list is now necessary for the six Classes dealt with in 1953. Five additional Classes, marked below with an asterisk, are also included in the new list. As there is still controversy regarding the status and arrangement of the major algal groups they are here given, as in Parke (1953), the rank of Class.