Photometric observations extending to 3.4 are presented for a sample of high-luminosity M stars. Intrinsic properties (color indices, effective temperatures, bolometric corrections) are determined for giants and supergiants; the supergiants are redder than the giants and have slightly lower effective temperatures. A weak luminosity distinction among the supergiants is seen in the infrared color data. The mean law of interstellar extinction, derived from the colors of distant reddened M supergiants, yields R = 3.6 + 0.3 (for M stars), and no large-scale anomalous regions are found. The extreme Population I character of M supergiants is confirmed by their spatial distribution. They are highly concentrated to the galactic plane and suggest the same spiral structure shown by other ear]y-type objects. The Ia supergiants show the strongest concentration in the plane-equal to that of the 0 and B stars.