Chromosomal DNA molecules of intracellular yeast-like symbiotes (YLS) of three species of rice planthoppers and two species of anobiid beetles have been separated by pulsed-field gel electrophoresis. Probable chromosome numbers of Nilaparvata lugens, Sogatella furcifera, and Laodelphax striatellus, were 4, 4, and 5, respectively, and tentative genome sizes were 17.3, 17.6, and 20.1 Mbp, respectively, based upon migration of individual chromosome-sized DNA relative to the size standards of Schizosaccharomyces pombe, Hansenula wingei, and Saccharomyces cerevisiae chromosomes. Chromosome numbers of Lasioderma serricorne and Stegobium paniceum were 11 and 15, respectively, and total genome sizes were 20.9 and 15.1 Mbp, respectively. The chromosomes carrying ribosomal RNA genes were identified by Southern blot analysis. Chromosomal organization and the genome size of YLS were similar to those of nonsymbiotic yeasts and fungi.