A study of the morphology and taxonomy of the species previously placed in Cystophora (Cystoseiraceae) results in the recognition of three genera: Cystophora J. Agardh, Acrocarpia Areschoug, and Caulocystis Areschoug. These genera are largely confined to the southern region of Austraiia, with six species of Cystophora occurring in New Zealand. Two eastern Pacific species previously placed in Cystophora (or Blossevillea) are now referred to other genera. Cystophora is characterized by bilateral and sympodial axes which are often flexuous, and monopodial laterals branched in various ways with terminal receptacles with conceptacles in rows; later axes arise in the axils of the laterals. The degree of axis flattening, branching of the laterals, form of receptacles and arrangement of conceptacles, and presence and form of vesicles are key morphological characters in distinguishing the species. Acrocarpia and Caulocystis have both radially and monopodially branched axes, Acrocarpia being tristichously branched without vesicles and Caulocystis radially branched with vesicles borne on the axes. The relationships of these genera with other genera of the Cystoseiraceae is given in a key to the genera, and keys are given for the species of the three genera. Evolution in Cystophora is considered to have been from species with unbranched laterals, through those with laterals branched in one plane to those with profusely and radially branched laterals.