The phylogeny of Ectocarpus and Kuckuckia strains representing widely separated populations ?om both hemispheres was infmed from sequence analysis of the internal transcribed spacers of the nuclear ribosomal DNA (ITS 1-5.8s-ITS 2) and Lhe spucer regton i n the plastid-encoded ribulos~bis-phosphate-carboxylase (RUBISCO) cistron (partial rbc L-spac:er+artial rbcS) . Both sequences resulted in matching phylogenies, with the RUBISCO spacer regzon being most i r~o~a t i v~ at the h e 1 of genera and .specie.s and the i n t m a l transm'bed spacer sequences at the level of species and populations. Three major clades were -formed f q strains previously described morphology and physiolob us Kuckuckia, E. fasciculatus, and E. siliculosus, con$min,g the validity of the.w taxa. Ectocarpus and Kuckuckia are regarded as sibling taxa with respect to the outgroup .species Feldmannia simplex, Hincksia mitchelliae, and Pilayella littoralis. The clade f m e d by .sexual E. siliculosus .strains and most asexual Ectocarpus strains was subdivided into several clades that are consistent with geographical races within E. siliculosus. The inferred phylogeny of Ectocarpus corresponds generally with resu1t.r from cro.ss-f)rrilization expm'ment.s, mmphology, and lipid analysis. A hypothesis on the origzn and dispersal of E. siliculosus suggests several natural dispersal events during periods of global cooling as well as recent and po.rsibly anthropogenic dispmsal event.$.