The response of chiasma frequency to a high dose of Actinomycin-D (60 μg/insect) was studied in male imagines of Schistocerca gregaria. Two stocks of locust were employed, one possessing a reduced chiasma frequency as a result of inbreeding. The high dose of AD caused rapid insect mortality, which limited the period of sampling to only 36 hours after injection. The chiasma frequencies of L and M bivalents were increased over the whole period of sampling in the inbred stock, and accurate estimates of meiotic timing place the period of sensitivity to the drug in late zygotene or pachytene. The other more heterozygous stock showed no response to AD, and in neither stock were the chiasma frequencies of the S bivalents affected. The possible mechanisms of the drug's action are briefly discussed.