The HOBAS aeration system was tested to compare changes in environmental and bacteriological parameters in ponds growing Penaeus monodon during a single production cycle. The stocking density in the aerated pond was doubled to 12 post-larvae (PL) m(-2) in contrast to the non-aerated pond with 6 (PL) m(-2). Microbial abundance in the ponds ranged between 10(5-6) cells ml(-1). Among the physiological groups of bacteria enumerated, the heterotrophs dominated with an abundance of 10(4) CFU ml(-1). Of the nitrogen and sulfur cycle bacteria, the nitrifiers flourished in the aerated pond and could maintain ammonia-N concentration within permissible levels. Bacterial activity also maintained sulfide concentrations at < 0.03 mg l(-1). Non-aerated conditions promoted denitrification maintaining nitrate concentration between 0.32 and 0.98 microM NO(3)(-)-N l(-1). However, a marked increase in ammonium content was observed in the non-aerated pond at the end of the culture period. Thus in high-density ponds, the aerators served to stimulate bacterial growth and activity which consequently maintained the quality of the water to match that of low-density ponds. Accordingly, these aerators could be effectively used to sustain higher yields. The effluent from the aerated pond is less likely to alter the redox balance of the receiving waters.