A comparative investigation into nuclear DNA content was carried out in the moss Physcomitruim pyriforme (n = 26 and 52) using 4-6- diamidino-2-phenylindole (DAM) as a flouorochrome. The mean relative DNA contents increase from haploid to diploid plants but they differ significantly from an expected 1 : 2 ratio in haploid and diploid races. Polyploidy has played a main role in generating the cytotypes. It seems likely that haploid and diploid races of P. pyriforme are of long standing autopolyploid.