Detection of HNCO in external galaxies.

We report the first detection of isocyanic acid (HNCO) from external galaxies through its 4_04_ - 3_03_ and 6_06_ - 5_05_ transitions. In NGC 253, if the source is extended, an observed [6_06_ - 5_05_]/[4_04_ - 3_03_] line ratio of ~2, close to the LTE value (2.25), suggests that the HNCO emission arises in a warm (T_ex_ >= 50 K) and optically thin medium with a column density, N_NHCO_ 1.8 10^14^ cm^-2^. Alternatively, in the case of an unresolved source, the data can be interpreted in terms of an optically thick source with a beam filling factor ~5 10^-3^ - 5 10^-4^. The excitation temperatures and densities are lower towards Maffei 2 and IC 342 (10-15 K, and ~2 10^13^ cm^-2^, respectively). No HNCO emission lines stronger than 10 to 20 mK have been detected in NGC 1068, M 82, NGC 3079 and M 83. We discuss the excitation of HNCO and its abundance in comparison with that predicted by chemical models.