We have cloned the three rows (thr) gene, by a combination of chromosome microdissection and P element tagging. We describe phenotypes of embryos homozygous for mutations at the thr locus. Maternal mRNA and protein appear to be sufficient to allow 14 rounds of mitosis in embryos homozygous for thr mutations. However, a small percentage of cells in syncytial blastoderm stage thr embryos sink into the interior of the embryo as if they have failed to divide properly. Following cellularisation all cells complete mitosis 14 normally. All cells become delayed at mitosis 15 with their chromosomes remaining aligned on the spindle in a metaphase-like configuration, even though both cyclins A and B have both been degraded. As cyclin B degradation occurs at the metaphase-anaphase transition, subsequent to the microtubule integrity checkpoint, the delay induced by mutations at the thr locus defines a later point in mitotic progression. Chromosomes in the cells of thr embryos do not undertake anaphase separation, but remain at the metaphase plate. Subsequently they decondense. A subset of nuclei go on to replicate their DNA but there is no further mitotic division.