Summary20 cases of irregular inheritance of phenogroups in the C system of cattle blood groups were used to deduce a partial genetic map of this system, taking into consideration the 11 internationally recognized antigenic factors and the 4 additional factors recently described by Grosclaude et al. (1980). This partial map bears resemblance to that established by Bouw et al. (1974) in Dutch cattle.The operational length of the DNA sequence coding for the C system was estimated to be 0.3 centimorgan, a value which is approximately half of that obtained for the B system by Grosclaude et al. (1979). It is concluded that the phenogroups of the C system, like those of the B system, are controlled by a cluster of loci.