  Using the IRAM 30m telescope, we report the detection of the CO(3--2),
CO(4--3), CO(5--4) and CO(6--5) lines in the gravitational lensed submm galaxy
SMM J16359+6612 at z=2.5. The CO lines have a double peak profile in all
transitions. From a Gaussian decomposition of the spectra we show that the CO
line ratios, and therefore the underlying physical conditions of the gas, are
similar for the blue and the redshifted component. The CO line Spectral Energy
Distribution (SED; i.e. flux density vs. rotational quantum number) turns over
already at the CO(5--4) transition which shows that the molecular gas is less
excited than in nearby starburst galaxies and high--z QSOs. This difference
mainly arises from a lower average H2 density, which indicates that the gas is
less centrally concentrated than in nuclear starburst regions in local
galaxies. We suggest that the bulk of the molecular gas in SMM J16359+6612 may
arise from an overlap region of two merging galaxies. The low gas density and
clear velocity separation may reflect an evolutionary stage of the merger event
that is in between those seen in the Antennae and in the more evolved
ultraluminous infrared galaxies (ULIRGs) like e.g. Mrk231.
