The caryological phases in the life history of Acrosymphyton purpuriferum (J. Ag.) Sjöst. (Rhodophyceae, Cryptonemiales) were investigated by measuring the relative amounts of DNA in nuclei of gametophytes and tetrasporophytes with the Feulgen microspectrophotometric technique. The relative amount of DNA in gametophyte nuclei is significantly smaller than that in tetrasporophyte nuclei. The average relative amount of DNA in tetrasporophyte nuclei is twice as large as that in gametophyte nuclei. It is concluded that in A. purpuriferum the gametophytes are haploid and the tetrasporophytes diploid.