[en] It is stated that interstellar isocyanide (HNC) was first identified on the basis of a calculated frequency for the J = 1 → 0 transition of the most abundant isotropic species H¹⁴N¹²C. The observed interstellar line was strong and widespread in the Galaxy. HNC is very short lived under terrestrial laboratory conditions and it has proved difficult to measure its rotational spectrum directly. The J = 0 → 1 transition has, however, been measured recently, and the frequency of the major isotope is in excellent agreement with that of the U90.7 emission line (90,663.6 MHz). Laboratory spectroscopy has also given the ¹³C isotope transition frequency. The present authors searched for and detected the HN¹³C J = 1 → 0 rotational spectral line at 87,090.6 MHz in the galactic molecular source W51 using the NRAO 11m. telescope. In addition the HN¹³C line was detected in some other sources. All of the spectra are discussed. (U.K.)