MOST present-day galaxies are not ’active’, in that they show no signs of continuing, high-energy events. The high energy output of an active galaxy is usually attributed to accretion of gas onto a massive black hole at its centre. Yet it remains an open question whether such black holes might exist at the centre of most large galaxies, and be undetected because no gas is presently being accreted to power a nuclear emission(1). Here we report the detection of an ultraviolet flare at the centre of NGC4552, an otherwise optically quiescent elliptical galaxy. The flare reached a luminosity about one million times that of the Sun, and probably arose from a mild accretion event onto a central black hole. The accreted gas could have come either from a star passing nearby, or from an interstellar cloud. This serendipitous discovery suggests that ultraviolet flares near the centres of galaxies may be common events, and offers a new way to search for black-hole-related activities in otherwise quiescent galaxies.