  We present simultaneous UV-G-R-I monitoring of 19 M dwarfs that revealed a
huge flare on the M9 dwarf 2MASSW J1707183+643933 with an amplitude in the UV
of at least 6 magnitudes. This is one of the strongest detections ever of an
optical flare on an M star and one of the first in an ultracool dwarf (UCD,
spectral types later than about M7). Four intermediate strength flares (Delta
m_UV < 4 mag) were found in this and three other targets. For the whole sample
we deduce a flare probability of 0.013 (rate of 0.018/hr), and 0.049 (0.090/hr)
for 2M1707+64 alone. Deviations of the flare emission from a blackbody is
consistent with strong Halpha line emission. We also confirm the previously
found rotation period for 2M1707+64 (Rockenfeller, Bailer-Jones & Mundt (2006),
http://arxiv.org/abs/astro-ph/0511614/) and determine it more precisely to be
3.619 +/- 0.015 hr.
