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Not AvailableThe bacteria involved in nitrogen and sulphur cycles and physicochemical characteristics of water and sediment in traditional, modified extensive and semi-intensive shrimp culture systems of West Bengal, India were monitored. The physicochemical parameters, except salinity and sediment pH, were observed to be well within the optimum levels recommended for shrimp culture. The levels of ammonia (NH3; PV0.05) and nitrite (NO2; Pb0.03) differed significantly among systems. The mean heterotroph counts were always close to or above 106/ml pond water and 106/g sediment. The counts of total heterotrophs and nitrate reducing bacteria (NRB) of the pond water of different culture systems differed significantly ( Pb0.01). While the counts of nitrite oxidizing bacteria (NOB; Pb0.001), nitrate reducing bacteria ( Pb0.01) and sulphate reducing bacteria (SRB; Pb0.03) in pond sediment of the three culture systems differed significantly. The mean counts of ammonia oxidizers increased with days of culture (DOC) in traditional and modified extensive systems, but they remained constant in semi-intensive pond sediment. The mean counts of nitrite oxidizers were generally high in water and sediment samples of modified extensive system. The mean sulphur oxidizing bacterial counts of pond water were higher in semi-intensive and modified extensive systems, which generally decreased with increasing DOC. The sulphate reducing bacterial counts of sediment increased with DOC in modified extensive and semi-intensive systems and vice versa inNot Availabl