Highly polyploid cells were detected in ganglia of Drosophila melanogaster larvae homozygous for the chbv40 mutation (78D) resulting from insertion of the P[lArB] construction. Frequency distribution of metaphases differing in chromosome number showed that the primary defect caused by the mutation involved formation of the unipolar chromosome spindle. This was confirmed directly by cytological analysis. Circular mitotic figures (CMF), i.e., circles of chromosomes with centromeres pointing inward and chromatid arms pointing outward, were also observed in neuroblasts of the homozygotes. Such structures are characteristic of polo and mgr mutations that alter the functions of the centrosome.