During the preparation of the Vascular Flora of the Marquesas Islands four new species of Bidens (Coreopsideae, Asteraceae) have come to light and are described herein: Bidens woodii W.L. Wagner, J.R. Clark & Lorence, sp. nov. from Ua Pou, and Bidens microcephala W.L. Wagner, J.R. Clark & Lorence sp. nov., Bidens evapelliana W.L. Wagner, J.R. Clark & Lorence, sp. nov., and Bidens wichmanii W.L. Wagner, J.R. Clark & Lorence, sp. nov. from the undercollected island of Fatu Hiva. In addition to these new species, we recognize the following six species of Bidens previously described from the Marquesas Islands: Bidens beckiana (F. Br.) Sherff (Eiao and Hatutaa), Bidens bipontina Sherff and Bidens cordifolia Sch. Bip. (both in Nuku Hiva), Bidens henryi Sherff (Hiva Oa, Tahuata), Bidens uapensis (F. Br.) Sherff (Ua Pou), and Bidens polycephala Sch. Bip. (Nuku Hiva, Ua Huku, Hiva Oa, Tahuata, and Mohotani). Two names are reduced to synonymy under Bidens polycephala: Bidens collina Degener & Sherff, syn. nov. and Bidens jardinii Sch. Bip., syn. nov. Bidens polycephala has the widest distribution of the Marquesan species on five of the islands and exhibits considerable variation.
Keywords: Asteraceae; Bidens; Coreopsideae; French Polynesia; Marquesas Islands; conservation.