  We present the first detection of molecular gas in the Cartwheel, the
prototype of a collisional ring galaxy formed in the head-on encounter of two
galaxies. Until now, only very little atomic gas and no CO had been detected in
the centre, where gas is theoretically expected to pile up. Using the Swedish
ESO Submm Telescope, we detected both 12CO(1-0) and (2-1) line emission towards
the central position. The line ratio and the line widths suggest that the
CO(2--1) emission is sub-thermal and that the CO(1-0) emission arises within
the central 22'' (13 kpc); it is probably associated with the inner ring and
nucleus. We infer a mass of molecular gas (H2) of 1.5 to 6 10^9 M_sol, which is
significantly higher than the approx. 10^8 M_sol of atomic gas within that
region. The low excitation of the gas, whether it is due to a low temperature
or a low density, is consistent with the weak star-forming activity observed in
the centre of the Cartwheel.
