at a range of radio frequencies (i.e. different redshifts).

OH megamasers may form a source of confusion in surveys designed to detect neutral hydrogen in normal galaxies through their 21 cm emission. High resolution radio mapping, optical spectroscopy and far infrared detection would remove the ambiguity.

The author is grateful to W.A. Baan, G.D. Bothun, A.G. de Bruyn, and J.M. van der Hulst for discussions and comments. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

Ashby M.L.N., Hacking P.B., Houck J.R., Soifer B.T., Weisstein E.W. 1996, ApJ 456, 428

Baan W.A. 1989, ApJ 338, 804

Baan W.A. 1997, in High Sensitivity Radio Astronomy, eds. Jackson, N., and Davis, R.J., Cambridge University Press, p. 73

Baan W.A., Rhoads J., Fisher K., Altschuler D.R., Haschick A. 1992a, ApJ 396, L102

Baan W.A., Haschick A., Henkel C. 1992b, AJ 103, 728

Boyle B.J., Shanks T., Peterson B.A. 1988, MNRAS 235, 935

Carlberg R.G., 1992, ApJ 399, L31

Clements D.L., Sutherland W.J., McMahon R.G., Saunders W. 1996, MNRAS 279, 477

Crane P.C., Napier P.J. 1989, in Synthesis Imaging in Radio 