An electronmicroscopical mapping of a number of regions of the polytene chromosomes of Ch. thummi salivary glands (3rd chromosome, right arm of the 1st chromosome, centromere regions, puffs 1-A2e, 1-A3ij, III-A5c and others) was done by the method of oriented ultrastructural sections of the unsquashed polytene chromosomes. The banding pattern on the electron micrograph was similar to the observed with the light microscope. The difference was that some doublets appeared as single cavity-containing bands with the double structure only in short regions under the electron microscope. It was also difficult to distinguish single bands in those regions where heavy adjacent bands were connected by dens, protrusions and anastomoses. These connections were most pronounced in the regions of the centromerers which had "spongy" appearance on the electron micrographs. These pictures may be connected with small interbands between heavy bands. Thin bands and some broad bands were frequently dotted. The puffs examined contained mainly RNP granules 200-400 A in diameter and RNP fibrils; BR-1 and BR-2 contained granules 500 A, RNP fibrils and smaller granules (200-400 A). BR and puffs were characterized by loop-like structures composed of granules arranged along the central DNP fibril. Only fibrils were presented in small interbands (0.05 mk), while larger interbands could include a small number of granules similar to those observed in puffs. It was found that centromere, telomeres and some heavy bands formed characteristic contacts with the nuclear membrane.