Context : V838 Mon erupted at the beginning of 2002 becoming an extremely luminous star with L=10 ^ { 6 } L _ { \sun } .
The outburst was followed by the most spectacular light echo that revealed that the star is immersed in a diffuse and dusty medium , plausibly interstellar in nature .
Low angular resolution observations of the star and its closest vicinity in the lowest CO rotational transitions revealed a molecular emission from the direction of V838 Mon .
The origin of this CO emission has not been established .

Aims : The main aim of this paper is to better constrain the nature of the CO emission .
In particular , we investigate the idea that the molecular emission originates in the material responsible for the optical light echo .

Methods : We performed observations of 13 positions within the light echo in the two lowest rotational transitions of ^ { 12 } CO using the IRAM 30 m telescope .

Results : Emission in CO J = 1– 0 and J = 2–1 was detected in three positions .
In three other positions only weak J = 1 - 0 lines were found .
The lines appear at two different velocities V _ { LSR } = 53.3 km s ^ { -1 } and V _ { LSR } = 48.5 km s ^ { -1 } , and both components are very narrow with FWHM \approx 1 km s ^ { -1 } .

Conclusions : The molecular emission from the direction of V838 Mon is extended and has a very complex distribution .
We identify the emission as arising from diffuse interstellar clouds .
A rough estimate of the mass of the molecular matter in those regions gives a few tens of solar masses .
The radial velocity of the emission at 53.3 km s ^ { -1 } suggest that the CO-bearing gas and the echoing dust are collocated in the same interstellar cloud .