  We report the first detection of X-ray emission from a brown dwarf in the
Pleiades, the M7-type Roque 14, obtained using the EPIC detectors on
XMM-Newton. This is the first X-ray detection of a brown dwarf intermediate in
age between ~12 and ~320 Myr. The emission appears persistent, although we
cannot rule out flare-like behaviour with a decay time-scale > 4 ks. The
time-averaged X-ray luminosity of Lx = (3.3 +/- 0.8) x 10^{27} erg/s, and its
ratios with the bolometric (Lx/Lbol = 10^{-3.05}) and Halpha (Lx/LHa = 4.0)
luminosities suggest magnetic activity similar to that of active main-sequence
M dwarfs, such as the M7 old-disc star VB 8, though the suspected binary nature
of Roque 14 merits further attention. No emission is detected from four
proposed later-type Pleiades brown dwarfs, with upper limits to Lx in the range
2.1-3.8 x 10^{27} erg/s and to log(Lx/Lbol) in the range -3.10 to -2.91.
