Context :

Aims : We study the distribution and physical properties of molecular gas in the disc around the T Tauri star IM Lup on scales close to 200 AU .
We investigate how well the gas and dust distributions compare and work towards a unified disc model that can explain both gas and dust emission .

Methods : ^ { 12 } CO , ^ { 13 } CO , and C ^ { 18 } O J =2–1 line emission , as well as the dust continuum at 1.3 mm , is observed at 1 \aas@@fstack { \prime \prime } 8 resolution towards IM Lup using the Submillimeter Array .
A detailed disc model based on the dust emission is tested against these observations with the aid of a molecular excitation and radiative transfer code .
Apparent discrepancies between the gas and dust distribution are investigated by adopting simple modifications to the existing model .

Results : The disc is seen at an inclination of 54 \degr \pm 3 \degr and is in Keplerian rotation around a 0.8–1.6 M _ { \odot } star .
The outer disc radius traced by molecular gas emission is 900 AU , while the dust continuum emission and scattered light images limit the amount of dust present beyond 400 AU and are consistent with the existing model that assumes a 400 AU radius .
Our observations require a drastic density decrease close to 400 AU with the vertical gas column density at 900 AU in the range of 5 \times 10 ^ { 20 } – 10 ^ { 22 } cm ^ { -2 } .
We derive a gas-to-dust mass ratio of 100 or higher in disc regions beyond 400 AU .
Within 400 AU from the star our observations are consistent with a gas-to-dust ratio of 100 but other values are not ruled out .

Conclusions :