IN some galaxies that have little gas remaining from the epoch of galaxy formation, the gas that is present rotates in the opposite sense to the stars(1-4). It is thought that this counter-rotating gas may result from the capture by a massive early-type galaxy of a gas-rich dwarf galaxy that was orbiting in the opposite sense to the main galaxy’s rotation; but as there are few examples of galaxies with counter-rotating gas, we have little information about the details of this process. Here we present optical spectra of the spiral(5,6) galaxy NGC3626, which clearly show the presence of counter-rotating ionized gas; combining these results with preexisting atomic hydrogen data(7) leads to an estimate of 10(9) M(.) for the mass of the gas. Spiral galaxies generally contain substantial amounts of gas left over from the formation epoch, which would be co-rotating with the stars; this gas should interact strongly with the counter-rotating gas on a relatively short timescale, causing it to fall towards the centre of the galaxy. We therefore propose that the counter-rotating gas in NGC3626 has been captured recently; the merger process is just beginning.