Chemical fractionation of the various isotopes of carbon mmonoxide has been studied in four isolated dark clouds. At positions away from the centers of three of the objects: B5, B335, and L1262: the abundance of /sup 13/CO relative to C/sup 18/O is at least a factor of 4 greater than at the central positions. This detection of carbon isotope fractionation is consistent with theoretical predictions of /sup 13/CO enhancement in the outer parts of cool clouds. Combined observations of the J=2..-->..1 and J=1..-->..0 transitions of several CO isotopes have enabled us to derive molecular hydrogen densities and to assess the effect of saturation and resonant scattering. Abundance ratios of the rarer isotopic species in the cloud centers have also been determined.