  We report high-resolution millimeter mapping with the IRAM Plateau de Bure
interferometer of rest-frame 335 micron continuum and CO(3-2) line emission
from the z=2.8 submillimeter galaxy SMMJ02399-0136. The continuum emission
comes from a ~3" diameter structure whose elongation is approximately east-west
and whose centroid is coincident within the astrometric errors with the
brightest X-ray and rest-UV peak (L1). The line data show that this structure
is most likely a rapidly rotating disk. Its rotation velocity of >420 km/s
implies a total dynamical mass of >3x10^11 solar masses within an intrinsic
radius of 8 kpc, most of which is plausibly in the form of stars and gas.
SMMJ02399-0136 is thus a very massive system, whose formation at z~3 is not
easy to understand in current CDM hierarchical merger cosmogonies.
