Spiral galaxies undergo chemical evolution as a result of the
 processing of material in the interior of stars, and the subsequent
 ejection of processed gas into the ISM. Thus the chemical state of
 a galaxy reflects its star formation history, material ejection and
 recycling phenomena, and the dynamical interaction of the different
 phases of the ISM. In particular the ratio of C to (13)C should
 evolve in time, as the abundance of the secondary species (13)C is
 enhanced over that of the primary species. Since the star formation
 rate is not homogeneous across a spiral galaxy (but is usually
 concentrated in its central region, at least in the initial stages
 of galactic evolution), in time a radial gradient of the isotopic
 ratios should be set up. A measurement of the gradient can thus set
 important constraints on the evolutionary history of a galaxy.
 We here propose to observe the 158 micron lines of ionized atomic
 carbon and its isotopic counterpart (13)CII in a range of positions
 along the Galactic plane to measure the distribution of the carbon
 isotopic ratio (13)C/C. This measure will be complementary to ground-
 based observations of molecular carbon species. Because of
 the long integration times required to detect the (13)CII lines,
 ISO is at present the only platform capable of carrying out these
 observations.