The molecular composition of alkyl substituted monocyclic aromatic hydrocarbons from asphaltite of the Ivanovskoe deposit was analyzed by means of the gas chromatography-mass spectrometry technique. Unlike most crude oils and natural bitumens, asphaltite is characterized by an unusual composition of practically all isobaric-homologous series of this class of hydrocarbons. The specific features of the composition of these series is a strong prevalence of C-12-C-23 1-alkyl-2,3,6-trimethylbenzenes with an irregular isoprenoid chain. These compounds are derivatives of isorenieratene, a carotenoid that occurs in anoxic photosynthetic green sulfur bacteria of the Chlorobiaceae family (Chlorobium genus). The ortho - isomers evidently prevail among C-13-C-25 alkyltoluenes, thus indicating a low level of catagenetic transformation of the asphaltite. n-Alkylbenzenes are practically absent from the monoalkylbenzene series; these are represented by the complete set of C-17 - C-19 phenylalkanes. Such compounds were detected for the first time in natural bitumens.