Observations of isotopic CO with C-12 (J = 1-0 and J = 2-1) and C-13 (J = 1-0) across the face of Cas A and at some adjacent positions are reported. All the CO-emitting gas lies in front of the continuum source. Although little spatial structure exists in the Local Arm velocity components, structure on a scale of 1-2 arcmin (1-2 pc) is found within the Perseus Arm components. It is argued that the CO must be clumped on a scale smaller than the 1.1-arcmin bandwidth of most of the observations and that the gas lies predominantly in dark rather than diffuse clouds. Estimates of gas densities are obtained that are several times smaller than those obtained by other authors from H2CO data.