Most radio sources are two-sided, but a minority appear onesided, 3C273 being the first-known and brightest example. There is no agreement1–4 on whether such sources are intrinsically one-sided, or are normal double sources, one half of which is hidden by Doppler effects. We report here new radio observations at 408 MHz of 3C273 which show that the brightness of the postulated counter-jet is 0.7c) into an ambient medium with number density <0.6m−3. Because several arguments suggest that such a density is implausibly low, the jet of 3C273 cannot be identified with the radio lobe of a normal double source. If the emitting regions are moving slowly the ejection from the nucleus is certainly to one side only. An alternative possibility is that the jet is moving relativistically and behaves as one of the fast beams in the beam-model of Blandford and Rees5.