Twelve root-tip metaphase cells of the diploid grassAegilops umbellulataZhuk. (2n= 2x= 14) were reconstructed in three dimensions from electron micrographs of serial thin sections. In ten of these cells each individual chromosome in the nuclei was identified by morphological criteria. No evidence for somatic association of centromeres of homologous chromosomes was found. However, with use of computer analysis, evidence was found suggesting: (i) that the centromeres are not arranged randomly on the metaphase plate but are ordered in two haploid sets; and (ii) that the order within each set is that predicted by a model based on spatial association of pairs of corresponding arms of most similar size on heterologous chromosomes.