1) C 18 O(1−0) OBSERVATIONS. We observed 13 points of C18O(1−0) (resolution 22″) around the nucleus of M82 and obtained typical main beam brightness temperatures of 20-50 mK. The intensity distribution reveals a clear double–peaked structure with the maxima seperated by 25″. The relative strength of the peaks w.r.t. the emission from the center is consistent with the presence of two point–like sources located at the peak positions. We compared our data with the 17″ CO(1−0) observations of Nakai et al. (1987) and found a strong contrast for the CO(1−0)/C18O(1−0) ratio with high values (≃30) in the center and low values (≃15) at the peak positions. This result is consistent with that of Loiseau et al. (1990) who also found with 12″ resolution for the ratios of CO(2−1)/13CO(2−1) minima at the ring locations, although not as strong as we found for the CO(1−0)/C18O(1−0) ratio. This high contrast indicates the presence of opically thick gas within the molecular ring and supports the view that the main part of the star forming process occurs in the ring.