THE detection and characterisation of hydrogen isocyanide, HNC, was for a long time a challenge to science, the first real progress coming from matrix-isolation studies of its infrared spectrum in 19631 and it has more recently been detected by its infrared chemiluminescence in certain flames involving atomic nitrogen2. In 1971 an interstellar line (U90.7) was attributed by radioastronomers3,4 to this molecule, the best estimate of the rest frequency of the line being6 90663.59 +/-0.15 MHz. The possibility that this line represents the J = 2 -->1 transition of Si C has also been advanced5. We have now succeeded in generating HNC in the gas phase and observing its microwave spectrum. The agreement between our observations of the frequency of the J = 1<--0 transition and the radioastronomical data render the identification of U90.7 as HNC secure.