The nucleotide sequences of the internal transcribed spacers (ITS1 and ITS2) of the nuclear ribosomal DNA were analyzed in species belonging to angiosperms, gymnosperms and pteridopthytes. It has been proved that these regions are a useful source of characters for phylogenetic studies in many angiosperm families. The size of the ITS1 and ITS2 is not exceeding 300 bp for both regions in the angiosperm species. The lengths of the ITSs of sixteen species of gymnosperms and five species of pteridophytes were estimated. The gymnosperms have ITS1 regions larger than those observed in the pteridophytes and angiosperms (c. 630–3160 bp versus 214–3160 bp). The ITS2 regions were 182–370 bp, with the exception ofGinkgo bilobawhere multiple–size lengths were recorded. By cloning or by PCR products, we have determined the complete nucleotid sequence of ITS regions from five gymnosperm species and three pteridophyte species. Sequence analysis showed the presence of five tandem arranged subrepeat with the size ranging between 219 bp and 239 bp within the 2631 bp ofPinus pinea,and three shorter tandem arranged subrepeats of about 70 bp in the 1112 bp of the ITS1 ofEphedra fragilis.DNA segments of 40–50 bp pyrimidine rich (about 90% were observed in the ITS1 ofGinkgo bilobaand at beginning of the five subrepeats ofPinus pinea.A 20 bp short sequence highly conserved in the ITS1 of the angiosperms species has been also found in the ITS1 of the shorter ribosomal genes ofPinus pineaand in the ITS1 ofTaxus baccataandEphedra fragilis.The secondary structures of some ITS1 and ITS2 are shown.