  We report the discovery of X-ray quasi-periodic oscillations (QPOs) at
frequencies of 3-4 mHz from a transient ultraluminous X-ray source (ULX)
X42.3+59 in M82. The QPOs are strong and broad and appear with weak or absent
red noise, and are detected only in Chandra observations when the source is
brighter than 10^40 ergs/s. The QPO behavior is similar to the type A-I QPOs
found in XTE J1550-564, which is a subclass of low frequency QPOs with
properties in between type A and B. Therefore, we identify the QPOs in X42.3+59
as of type A or B, and rule out the possibility of type C. With this
identification, the mass of the black hole in X42.3+59 can be inferred as in
the range of 12,000-43,000 solar masses by scaling the QPO frequency to that of
the type A/B QPOs in stellar mass black holes. Cool disk emission is detected
in one Chandra observation, and the disk inner radius suggests a similar black
hole mass range. Black holes of such a high mass are able to produce an energy
output in a manner similar to X42.3+59 by accreting from the interstellar
medium directly.
