The localization of the absorption peaks of toluidine blue-stained heterochromatin (Ho and Hs) and euchromatin (Eo and Es) of the Malpighian tubules of the blood-sucking hemipteran, Triatoma infestans Klug was studied cytophotometrically (Ho and Eo: hetero- and euchromatin of nuclei with one single chromocenter; Hs and Es: hetero- and euchromatin of nuclei with several chromocenters). shifts of the absorption peaks to longer wave-lengths were found for the euchromatin fractions and Hs after RNAse treatment. The differences in the localization of absorption peaks after RNAse digestion when comparing the various chromatin types with one another are assumed to be due to differences in the number and proximity of DNA phosphates unattached to proteins and hence available to dye binding. The localization of the absorption peak of Ho at a relatively short wavelength (lambda = 570 nm) as compared with those for Hs, Eo, and Es, is attributed to a special state of chromatin condensation and practically absence of RNA in Ho. Also a certain removal of histones (probably, Hl) by the acetic ethanol "fixation" is speculated to play a role in the absorption curve properties of Ho subjected to TB staining.