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Molecular data obtained by a procedure for extracting PCR-amplifiable nuclear and chloroplast DNA from old and formalin-fixed red algal herbarium specimens were used to elucidate problems in the systematics of Pacific Gigartinaceae. Correspondence between nucleotide sequences of the internal transcribed spacer 1 region or the RUBISCO spacer from type specimens and modern collections supports the following conclusions. (1) The type of Fucus cordatus Turner, now Iridaea cordata (Turner) Bory, came from the southern hemisphere (probably from Isla de los Estados, Argentina) rather than from Banks Island, B.C., Canada. (2) The type of Iridaea heterocarpa P. et R. [Mazzaella heterocarpa (P. et R.) Fred.] represents the tetrasporangial phase of a species of Chondrus, possibly C. crispus Stackh. (3) The types of Iridaea lilacina P. et R., I. phyllocarpa P. et R., and Iridophycus furcatum S. et G. represent a single species from Alaska, Mazzaella phyllocarpa (P. et R.) Perest., currently but incorrectly called M. heterocarpa. (4) The type of Iridophycus oregonum Doty represents the tetrasporangial phase of the species from southern Alaska to southern California known incorrectly as M. heterocarpa. (5) Mazzaella splendens (S. et G.) Fred. is more closely related to M. linearis (S. et G.) Fred. than it is to M. flaccida (S. et G.) Fred. (6) Iridophycus coriaceum S. et G. is conspecific with M. splendens, whereas Rhodoglossum coriaceum E.Y. Dawson is an independent species: Mazzaella coriacea (E.Y. Dawson) Hughey. (7) Iridaea cornucopiae P. et R. is conspecific with Mazzaella laminarioides (Bory) Fred., and the type probably came from Chile rather than from the North Pacific. (8) Plants attributed to Iridaea cornucopiae in Pacific North America are referable to Mazzaella parksii (S. et G.) comb. nov. (9) Rhodoglossum parvum G. M. Smith et Hollenb. is an independent species: Mazzaella parva (G. M. Smith et Hollenb.) comb. nov. (10) Grateloupia squarrulosa S. et G., Grateloupia johnstonii S. et G., and Gigartina pectinata E.Y. Dawson represent a single species: Chondracanthus squarrulosus (S. et G.) comb. nov.