A rapid and sensitive procedure for the determination of methylcytosine in cucurbit DNAs using reversed-phase high-performance liquid chromatography is described. The results showed ca. 3.2-5.6% of methylcytosine in total DNA, i.e., about 2&30% of the cytosine residues appeared to be methylated. These results are discussed taking into consideration the influence of degree of methylation on the G + C determination of DNAs.