The development of the Mobility Inventory for Agoraphobia (MI), a 27-item inventory for the measurement of self-reported agoraphobic avoidance behavior and frequency of panic attacks, is described. On this instrument, 26 situations are rated for avoidance both when clients are accompanied and when they are alone. These two conditions were found to be only moderately correlated and are thus analyzed separately, as is the item on frequency of panic attacks. Studies are reported which support the reliability and the concurrent and construct validity of these three measures, using comparison samples of agoraphobic, socially phobic and normal Ss. The MI appears to be a sound instrument, with which a broad range of situations troublesome to agoraphobic clients can be surveyed, and should prove useful for treatment planning and research.