Observations were made on chromosome numbers of isolates of Porphyra linearis (Rhodophyta) from western Ireland and Maine, USA. Chromosome numbers differed between specimens from East and West Atlantic populations. In Irish isolates the haploid chromosome number was n = 4 for all the specimens examined. Conchocelis filament cells and conchosporangial cells were diploid with 2n = 8, but zygotosporangial cells and some conchospores released had a diploid chromosome number which varied between 8 and 10. By contrast, in isolates from Maine, USA, the haploid chromosome number was mainly 3, with variations between 4 and 2. The loss or gain of one chromosome, probably caused by aneuploidy in the US isolates, could be explained as a step on the evolution of the chromosome number to 4. The results suggest that the P. linearis populations from the East and West Atlantic represent two separate entities.