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We present a fully-sampled map of M31 in the CO (J = 2 $\rightarrow$ 1) transition, for a 2' $\times$ 3' section of the southwest arm-interarm region centered at major and minor axis coordinates (-42.0,+9.0), made with the NRAO$^{\ast}$ 12 meter telescope. The map is sampled at intervals of one-half beamwidth, with an angular resolution of 30", or $100 \times 460$ pc along the M31 major and minor axes, respectively. We observed 101 positions, with an rms noise level less than 20 mK (on the ${\rm T_{R}}^{\ast}$ scale) at most positions, and with an effective velocity resolution of $3.9$ km/s.
We identify a distinction between the CO spectra in the section of our region covering the arm, as defined by HII regions, and the spectra off the arm. Along the arm, the CO spectra are characterized by strong narrow lines; we interpret these as giant molecular clouds with masses of a few $\times 10^{5} {\rm M}_{\odot}$ . The spectra away from the arm are much weaker, with masses of a few $ \times 10^{4} {\rm M}_{\odot}$ . Also, there is a velocity shift in the arm spectra of about $15$ km/s towards more negative radial velocities. We will discuss in detail the difference in character between the arm and interarm emission, in particular the abruptness of the velocity shift in the molecular material along the arm.
\noindent D. Mizuno was aided by the travel support program of the Astronomical Society of New York.
\noindent ${\rm {}^{\ast}}$ The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation.
