In species of the red algal genus Laurencia the life cycle is isomorphic, with haploid gametophytic and diploid tetrasporophytic thalli similar or identical in shape. Identical total nuclear DNA contents are postulated in the literature for homologous somatic thallus cells of both generations. However, in the closely related genus Osmundea differences in DNA content were observed between some homologous somatic cells of the isomorphic gametophytes and tetrasporophytes. To find out if differences between nuclear gametophytic and tetrasporophytic DNA contents also exist in Laurencia, comparative studies were carried out in two species of this genus. Microfluorometric measurements of nuclear DNA contents in the uninucleate cells of the central filaments in distal portions of thalli of Laurencia majuscula and Laurencia sp. were up to 64C for tetrasporophytes and up to 32C for gametophytes. Most probably nuclei of apical cells in G 2 phase were 128C in tetrasporophytes and 64C in gametophytes. Nuclei of the multinucleate medullary, cortical and epidermal cells were mostly 8C and 4C in tetrasporophytes and gametophytes. Flow charts for nuclear DNA contents starting with apical cells are presented. Average C-levels of gametophytic nuclei were lower than those of homologous tetrasporophytic nuclei, reflecting the respective haploid and diploid states in these isomorphic generations. Because homologous gametophytic and tetrasporophytic cells had identical numbers of nuclei, total nuclear DNA contents per cell were also different. Nuclear DNA content ranges in multinucleate cells were 1C -16C in gametophytes and 2C -32C in tetrasporophytes. So, some tetrasporophytic nuclei had C-values identical to those of some gametophytic nuclei. Tetrasporophytic and gametophytic nuclei with identical C-levels are considered to be responsible for isomorphy of the two generations. We suggest that low C-levels in trichoblasts of gametophytes and high C-levels in cortical and epidermal cells of tetrasporophytes control the development of gametangia and tetrasporangia, respectively. Previous reports that nuclear DNA contents in homologous gametophytic and tetrasporophytic cells are identical in Laurencia are not confirmed.