Restriction fragment length polymorphisms (RFLPs) of chloroplast DNA (cpDNA), mitochondrial DNA (mtDNA), and ribosomal DNA (rDNA), were used to estimate and compare the extent of diversity among the organellar and nuclear genomes, and to infer species relationships. Eight cultivated turfgrass species and subspecies were used in this study including: Festuca rubra spp, rubra, F. rubra spp commutata Gaud., F. rubra spp trichophylla Gaud., F. longifolia Thuill., F. ovina L, F. arundinacea Shreb, Lolium perenne L., and Poa pratensis L. Genomic DNA from tissue samples of 208 cultivars representing the eight turfgrasses was digested with each of four restriction enzymes: Hind III, Bam HI, Eco RI, and Xba I, and probed with a set of ten barley cpDNA clones, nineteen wheat mtDNA clones, and one wheat rDNA clone. The degree, type, and distribution of diversity detected within and between these species and subspecies were compared by RFLP analysis. Relative phenotypic diversity in the cytoplasmic and nuclear genomes was evaluated using Shannon’s information statistic. Genetic similarities used for computing species relationships were based on the proportion of shared RFLP fragments. Substantial inter- and intra-specific nuclear and cytoplasmic DNA variation was detected with RFLP markers in the eight turfgrass species and subspecies. Comparison of phenotypic diversity estimates indicates that, in general, the highest level of variation was detected by rDNA, followed by mtDNA, and the lowest was by cpDNA. The high variability in rDNA indicates that rDNA in these species evolves at a faster rate than both cpDNA and mtDNA, and cpDNA evolves at a slower rate than mtDNA. Species relationships derived based on the data of the three genomes indicate that the five fine fescues are clustered in the same group in agreement with the traditional classification. Relationships among the eight turfgrasses based on mtDNA data are in accordance with those from cpDNA. Furthermore, results from these analyses indicate that F. arundinacea and L. perenne are closely related to each other, and P. pratensis has a rather low degree of relationship to any of the turfgrasses studied. This is the first study where the data from three genomes have simultaneously been used to address genetic variation and species relationships in plants. The results of this study further indicate that RFLPs of cpDNA, mtDNA and rDNA are useful markers for species and variety identification, and as criteria in germplasm collection as well as in the elimination of duplicates accessions in germplasm banks. / Ph. D.