The effect of torsion on the flow in a helical tube of circular cross-section is experimentally investigated over a range of Reynolds numbers from about 500 to 20000. Three cases of dimensionless curvature fi were studied, i.e. fi = 0.01, 0.05 and 0.1. The torsional parameter/30, which is defined as /30 = z/v/~ with dimensionless torsion r, was considered for seven cases between 0.45 and 1.72. The results reveal rather large effect of torsion on the flow: the friction factor of the flow at a fixed ~ deviates from that of the toroidally curved tube without torsion as/30 increases, and it decreases toward that of the straight tube as/30 further increases. It is also found that the torsion has a destabilizing effect on the flow. The critical Reynolds number at the onset of turbulence varies with/30 and has a minimum at/30 -~ 1.3. The minimum critical Reynolds number at 6 = 0.1 is about 800.