Male individuals ofAsellus aquaticuswere injected with BrdU and Hoechst 33258; gonad preparations were set up at various time intervals after treatment. Both substances Acted in the first 4h of treatment, outside of S phase, interfering with the normal mitotic and meiotic process of chromosomal condensation. A very slight banding appeared on the mitotic metaphase chromosomes 48 and 65h after treatment; this being the only effect obtained which is likely due to the action of the two substances during S phase.