The freshwater red alga, Batrachospermum helminthosum, is distributed primarily in streams of eastern North America. Although it is often an abundant and conspicuous component of the algal flora for many streams, little is known about the genetic relationship among populations throughout its range. The purpose of our study was to elucidate geographic patterns among populations to gain insight into the biogeographic distribution of this species and possible modes of dispersal. Individuals were sampled in 13 stream segments as follows: four locations in Ohio, two locations in Michigan and one location each in Indiana, North Carolina, Tennessee, Louisiana, Rhode Island, Massachusetts and Connecticut. The mitochondrial spacer region (371bp) between the COX2 and COX3 genes was chosen because it is variable among individuals. This region was sequenced for 15 individuals from each location. Thirteen haplotypes were identified among the 13 locations with most locations having one or two haplotypes. Two of the haplotypes were dominant being recorded from 6 of the streams. The Indiana, Louisiana and first Michigan population had a unique haplotype. The second Michigan and fourth Ohio populations shared a unique haplotype. The Tennessee population had 4 haplotypes, 3 of which were closely related but the fourth haplotype was not and linked to the haplotypes from Michigan and Ohio. The relationships among these populations are very complex but there may have been a recent dispersal event via birds among various streams.

Key words: Batrachospermum helminthosum, COX mitochondrial DNA, freshwater streams, rbcL, Rhodophyta