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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 of 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 among the CO spectra in the arm, as defined by HII regions, the spectra off the arm, and the spectra in the `transition' region. Off the arm the spectra are weak, corresponding to molecular clouds with masses of a few $ \times 10^{4} {\rm M}_{\odot}$. On the arm, the spectra are characterized by isolated strong narrow lines shifted by about $15$ km/s towards more negative radial velocities. We interpret these as giant molecular clouds with masses of a few $\times 10^{5} {\rm M}_{\odot}$ . The spectra in the `transition' region exhibit a double-peaked structure, with a component at the shifted arm velocity as strong as or stronger than the arm spectra, and a weaker component at the interarm velocity. These spectra have a similar appearance at many adjacent positions, suggesting a large number of small clouds rather than a few large clouds. Also, this transition region corresponds to a prominent dust lane and peaks in IRAS HiRes 60$\mu {\rm m}$ and 100$\mu {\rm m}$ emission. The effects of the spiral arm perturbation on molecular cloud formation and star formation will be discussed.
\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.
