We present measurements of the [ O ii ] emission line width for a complete sample of 24 blue field galaxies ( 21.25 < B < 22 , B - R < 1.2 ) at \langle z \rangle \sim 0.25 , obtained with the AUTOFIB fibre spectrograph on the Anglo-Australian Telescope ( AAT ) . Most emission lines are spectrally resolved , yet all have dispersions \sigma _ { v } < 100 { km~ { } s } ^ { -1 } . Five of the 24 sample members have [ O ii ] doublet line flux ratios which imply gas densities in excess of 100 cm ^ { -3 } . The line emission in these galaxies may be dominated by an active nucleus and the galaxies have been eliminated from the subsequent analysis . The remaining 19 linewidths are too large by a factor of two ( 7 \sigma significance ) to be attributed to turbulent motions within an individual star forming region , and therefore most likely reflect the orbital motion of ionized gas in the galaxy . We use Fabry–Perot observations of nearby galaxies to construct simulated datasets that mimic our observational setup at z \sim 0.25 ; these allow us to compute the expected distribution of ( observable ) linewidths \sigma _ { v } for a galaxy of a given “ true ” ( optical ) rotation speed v _ { c } . These simulations include the effects of random viewing angles , clumpy line emission , finite fibre aperture , and internal dust extinction on the emission line profile . We assume a linewidth–luminosity–colour relation : { log } \Bigl [ v _ { c } \bigl ( M _ { B } ,~ { } B - R \bigr ) \Bigr ] ~ { } = ~ { } { log } \ > [ v _ { c } ( - % 19 , 1 ) ] - \eta \bigl ( M _ { B } +19 \bigr ) + \zeta [ ( B - R ) -1 ] and determine the range of parameters consistent with our data . We find a mean rotation speed of v _ { c } ( -19 , 1 ) = 66 \pm 8 { km~ { } s } ^ { -1 } ( 68 % confidence limits ) for the distant galaxies with M _ { B } = -19 and B - R = 1 , with a magnitude dependence for v _ { c } of \eta = 0.07 \pm 0.08 , and a colour dependence of \zeta = 0.28 \pm 0.25 . Through comparison with several local samples we show that this value of v _ { c } ( -19 , 1 ) is significantly lower than the optical rotation speed of present-day galaxies with the same absolute magnitude and rest frame colour ( \approx 105 { km~ { } s } ^ { -1 } ) . The most straightforward interpretation is that the distant blue , sub-L _ { * } galaxies are about 1.5 mag brighter ( and \geq 0.8 mag brighter at 99 % confidence ) than local galaxies of the same linewidth and colour .