AbstractThere are three cytotypes ofAtrichum undulatumin the British Isles withn= 7, 14 and 21. The triploid is the most frequent (56.71% of populations sampled), followed by the diploid (32.52%) with the haploid the rarest (10.77%). There is a south to north trend of a decrease in the proportion of haploid populations and an increase in triploids. The cytotypes are indistinguishable morphologically both before and after cultivation under uniform conditions and cannot be accorded taxonomic recognition.