Whole-mount electron microscopy of metaphase mouse L-cells showed that in many cases the region of the centromere was located at one extreme end of the chromosome with no evidence for short arms of any significant size. In such telocentric chromosomes there was only a single area of chromatid association. By contrast, the region of the centromere in mouse metacentric chromosomes was much longer. When well dispersed, it was seen to consist of two distinct areas of chromatid association, each of which was comparable in length to that of the telocentric chromosomes. In the region between these two areas there was a decrease in the density of the chromatin fibers. A similar quadripartite morphology was seen in human, Chinese hamster, and sheep metacentric chromosomes. These findings are most simply interpreted by the proposal that the quadripartite centromere of the metacentric chromosomes has resulted from the fusion of two telocentric chromosomes with bipartite centromeres.