AFTER the first detection of the 616-523 transition of water vapour in an external galaxy by Churchwell et al.1 in M33, other detections were reported by Lépine and dos Santos2 in NGC253, and by Huchtmeier et al.3 in M33 and IC342. We report here the detection of H2O maser emission from NGC4945, a bright edge-on spiral galaxy in the Centaurus group. This is intrinsically the most powerful H2O maser yet detected in any astronomical source. The search for H2O emission in NGC4945 was motivated by the previous detection of OH and H2CO absorption by Whiteoak and Gardner4 and Gardner and Whiteoak5, and also by the similarity of this galaxy to NGC253. The observations were made on 20-22 September 1978, with the 13.7-m Itapetinga radiotelescope equipped with a ruby maser amplifier with an apparent SSB system temperature of about 150 K. The signal was analysed by a 46-channel, 100-kHz (1.35 km s-1) resolution filter bank, and observations were made with the filter bank centred at different frequencies to obtain greater velocity coverage. The 4' beam was pointed towards the nucleus of the galaxy. The overall dimensions of the galaxy are 17' × 3'. The telescope was beam switched at a rate of 100 Hz. The beams were separated by 9.4 arc min and the reference beam was displaced in azimuth, which did not correspond to a fixed direction with respect to the equatorial plane of NGC4945. Antenna temperatures were corrected for atmospheric attenuation and random attenuation using the standard procedure at Itapetinga6 and then converted to flux units, the error in the flux scale being ~10%.