THE 616-523 transition of water vapour at 22.235 GHz was observed for the first time in an external galaxy by Churchwell et al.1 in M33 with the 100-m Effelsberg radio telescope in late 1976, after some previous unsuccessful searches2,3. We report here the second detection of H2O emission from an external galaxy, NGC253, a large edge-on spiral galaxy situated at 3.4 Mpc (ref. 4), about five times farther away than M33. Previous observations of other molecules pointed NGC253 as a good candidate for H2O emission: OH was detected by Weliachew5 and confirmed by other workers6,7; CO was detected by Rickard et al.8 and by Soloman and Zafra9, and H2CO was detected by Gardner and Whiteoak10. The time schedule of the 13.7-m Itapetinga radio telescope during 1977 May permitted a very long integration to be performed on this object, which is observable more than seven hours a day at elevation angles greater than 30°. The observations were made with a double sideband balanced mixer receiver of about 1,000 K system temperature and a 46 channel, 100 kHz resolution filter bank. Beam switching at a frequency of about 100 Hz was used, the main beam and then the 9' reference beam being pointed at the nucleus of NGC253 for alternate intervals of 1 min. The sign of the recorded signal was changed every minute by the data acquisition computer, so that possible zero-level offsets of the channels were eliminated. One-hour integrations were made with the filter bank centred alternatively at three different frequencies in order to obtain greater velocity coverage, until a total on-source integration time of 21 h was completed at each frequency band. The data analysis program corrected the antenna temperatures for atmospheric attenuation as a function of zenith angle exp (-τ sec z) and then investigated the distribution of the results of the one-hour observations for each channel, giving the standard error of the mean and rejecting data more than 3 σ different from averaged values; the channels were subsequently averaged two by two, simulating a 200 kHz resolution system.