  We report the detection of H{$\alpha$} emission in the T dwarf (methane brown
dwarf) 2MASSW J1237392+652615 over three days using the Keck Low Resolution
Imaging Spectrograph. The measured line flux, log(L$_{H{\alpha}}$/L$_{bol}$) =
$-$4.3, is roughly consistent with early M dwarf activity levels and
inconsistent with decreasing activity trends in late M and L dwarfs. Similar
emission is not seen in two other T dwarfs. We speculate on several mechanisms
that may be responsible for emission, including a strong magnetic field,
continuous flaring, acoustic heat generation, and a close ($a$ $\sim$ 4 - 20
R$_{Jup}$) interacting binary, with the cooler component overflowing its Roche
lobe. We suggest that the M9.5Ve PC 0025+0447 could be a warm analogue to 2MASS
J1237+65, and may be powered by the latter mechanism.
