Context : Water fountain stars are very young post-AGB stars with high velocity water maser jets .
They are the best objects to study the onset of bipolar jets from evolved stars due to their young dynamical ages .
However , none of them have been observed in any thermal lines .

Aims : Search for CO lines in the water fountain star IRAS 16342-3814 and investigate the properties of its thermal gas .

Methods : The proximity , peculiar stellar velocity and high Galactic latitude of IRAS 16342-3814 make a single dish observation possible .
We use the Arizona Radio Observatory 10m telescope to observe the CO J=2 - 1 line and compare the line parameters with that of masers .

Results : We report the detection of ^ { 12 } CO and ^ { 13 } CO J=2 - 1 lines from IRAS 16342-3814 .
The inferred ^ { 12 } CO mass loss rate is an order of magnitude lower than the infrared and OH mass loss rates , indicating a very cold and thick O-rich circumstellar envelope around the star .
We also find a ^ { 12 } CO expansion velocity of V _ { exp } = 46 \pm 1 km s ^ { -1 } that is too high for an AGB wind and confirm the systemic velocity of 44 \pm 1 km/s .
In addition we measure a very low ^ { 12 } CO/ ^ { 13 } CO line ratio of 1.7 .

Conclusions : The first detection of CO lines has provided a new way to investigate the water fountain stars .
Given the high expansion velocity of the CO gas and its relation to maser velocities , we infer that the CO emission region is co-located with the OH mainline masers in the warm base of the optical bipolar lobes , while the high velocity OH 1612 MHz and H _ { 2 } O masers are located in the side walls and at the farthest ends of the bipolar lobes , respectively .
Further observations are highly desired to understand the very low ^ { 12 } CO/ ^ { 13 } CO line ratio .