  We note that the recently discovered 450 Hz frequency in the X-ray flux of
the black hole candidate GRO J1655-40 is in a 3:2 ratio to the previously known
300 Hz frequency of quasi-periodic oscillations (QPO) in the same source. If
the origin of high frequency QPOs in black hole systems is a resonance between
orbital and epicyclic motion of accreting matter, as suggested previously, the
angular momentum of the black hole can be accurately determined, given its
mass. We find that the dimensionless angular momentum is in the range
$0.2<j<0.65$ if the mass is in the (corresponding) range of 5.5 to 7.9 solar
masses.
