The GC and AT fractions (GC content is 55.2 and 36.2 mole %, respectively) were isolated from wheat germ DNA by the thermal fractionation method (sequential thermal DNA denaturation followed by separation of denatured and undenatured DNA molecules by hydroxyapatitie chromatography). The amount of each of the two fractions isolated corresponds to about 25% of the total DNA. The 5-methylcytosine (m5C) content in the GC fraction (9.2 mole %) is 4.6 times higher than in the AT fraction. The DNA fractions isolated are very different in the level of methylation of cytosine residues (m5C/C+m5C ratio differs 3 times), in the amount of respective pyrimidine isostichs and in the renaturation kinetics. In contrast to the AT fraction, in the GC fraction the amount of dipyrimidine isostichs is higher than monopyrimidine ones. The AT and GC fractions isolated contain unique, moderately and highly reiterated nucleotide sequences. About 90% of the GC fraction is represented by reiterated sequences. The isolated (from the GC fraction) highly reiterated sequences (GC fraction I, Cot = 2.8 X 10(-2)) are very similar to the original GC fraction in the GC and m5C contents as well as in the amount of m5C in the respective pyrimidine isostichs. GC fraction I differs from the total GC fraction in a lower pyrimidine clustering degree. In total DNA and in GC and AT fractions m5C is found in mono- (34-40% of total m5C), di-(26-30%) and in all longer pyrimidine isostichs. It is suggested that in plants there exist many types of DNA sequences to be methylated as well as many respective DNA methylases.