pita^k14408 mutant larvae develop melanotic tumours, their imaginal discs are greatly reduced in size and they fail to develop to the pupal stage. Mitotic figures in pita^k14408 mutant larval brains show highly condensed chromosomes, premature separation of sister chromatids and irregular chromatin condensation or chromosome breaks. Anaphase figures are abnormal, showing overcondensed chromatin, lagging chromosomes and chromosome bridges. Mitotic spindles are affected to varying degrees; some cells have highly overcondensed chromosomes associated with disintegrating and frequently anastral spindles, some have hyper-condensed and misaligned chromosomes, while others are seemingly normal. However, the mitotic index of these cells is not significantly increased compared to wild-type cells. pita^k14408/Df(2R)bw5 mutant larval brains also show the above defects. FACS analysis of S2 cells shows that the proportion of pita^k14408 cells in S and G2/M phase is approximately two-fold greater than control cells. As the mitotic index is not significantly changed, the increase in G2/M cells must represent cells that are delayed in G2. BrdU incorporation into pita^k14408 larval brains is almost completely abolished, showing that replication of genomic DNA is perturbed. DNA replication appears to also be perturbed in endocycling tissues as pita^k14408 salivary glands are much smaller than wild-type glands and their polytene chromosomes appear highly under-replicated and lack the normal banding pattern.