Mitosis takes place correctly in cycles 1-14. Propidium iodide staining reveals that in mitosis 15 abnormalities become evident as areas of cells delayed in mitosis, that subsequently show disorganised chromatin. Careful analysis of syncitial blastoderm embryos shows that a small percentage of nuclei lose their association with the cortex during cycles 11 to 13 and sink into the interior of the embryo. Analysis with propidium iodide, and immunostaining for microtubules, Cen185, CycA and CycB shows that the order of entry into mitosis of each domain is wild type at cycle 14, and BrdU labelling showed that DNA synthesis immediately after mitosis 14 is normal. In cycle 15 the mitotic domains of thr embryos contain mostly metaphase figures with no anaphase figures that characterize wild type: thr embryos show mitotic domains coalesced into large patches. The thr mutation delays metaphase in terms of chromatid separation, even though, by the criterion of cyclin B degradation, the metaphase to anaphase transition proceeds normally.