The chemical nerve agent sarin (o-sopropyl methylphosphonofluoridate) was hydrolyzed at 7.2 wt % in aqueous sodium hydroxide yielding primarily o- isopropyl methylphosphonic acid (IMPA). This hydrolysate was diluted and fed as sole carbon source to activated sludge in an aerobic sequencing batch reactor. Feed concentrations ranged from 0.25% (initial) to 0.5% (final). The reactor was operated essentially on a 15-day hydraulic residence time. Overall chemical oxygen demand removal efficiency was 86.2%, and the IMPA in the feed was quantitatively converted to methylphosphonic acid (MPA). MlCROTOX and Daphnia magna aquatic toxicity testing showed the effluent to be of very low toxicity to aquatic test organisms. The final MPA product was effectively absorbed by Phoslock, which is a modified clay.