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The nuclear quadrupole coupling constants of the nitrogen atom in pyrrole have been redetermined by analysis of the hyperfine structure for several low J rotational transitions. The resulting values arex"" = 1'400±O'OO8 MHz and Xbb = 1·300±O·OO8 MHz. Small modifications to the previously reported rotational constants are given.Recent interest in the discovery of molecules in interstellar space led us to investigate the possibility of finding pyrrole, a simple heterocyclic compound containing atoms of only carbon, hydrogen and nitrogen. The importance of pyrrole in relation to the exobiology of porphyrins has recently been emphasized and a plea made for a search in the region of Sagittarius (Hodgson 1971). As a preliminary to the interstellar search, it was necessary to study several low J absorptions ofpyrrole in the laboratory. The microwave spectrum, including the nitrogen quadrupole analysis,was published by Nygaard et al. (1969) but during our study of the Ill-ItO transition it became evident that the multiplet structure of the absorption could not be accurately predicted using the published microwave data. We therefore decided to study several multiplets of pyrrole in order to obtain more accurate values of the nitrogen quadrupole coupling constants.Pyrrole (Koch-Light. pure grade) was used without further purification. Spectra were recorded using a conventional 5 kHz Stark-modulated G band spectrometer. Frequency measurements were made with a Hewlett-Packard 5246L electronic counter which was calibrated periodically against a Sulzer 2· 5C laboratory standard. Absolute frequencies of hyperfine components are quoted to an accuracy of ± 10kHz whereas frequency differences, between various components of the multiplet, can be determined to within ± 5 kHz. The pyrrole sample was contained in a dry-ice cooled absorption cell at pressures of approximately 1 J.tmHg (0'13 Pa). Time constants of the orders of 1 s were used whilst scanning the multipiets.A maximum resolution of 20 kHz half-width was achieved for the l 1C 1 10 transition. Substitution of the frequency separation between successive members of this multiplet into the well-known relationship of Casimir (1936)