The acute toxicity and the effect of sub-lethal concentrations of calcium carbonate on the morphology of M. macrobrachion were investigated under laboratory conditions. The prawns were exposed at 0, 160, 320, 640, 1280, 2560 and 5120 mg/l of CaCO3 using static renewal bioassay for 96 hours for the acute toxicity test. The prawns were also exposed for the chronic test at 0, 20.0, 40.0, and 80.0 mg/1 for a period of two weeks. Mortality occurred in the bioassay tanks at random and the bioassays were monitored throughout the exposure period. Results showed that exposed prawns were not significantly impacted when compared with the control group and correlations were significant at P<0.05. There were no mortalities in the tanks with the highest concentrations while one death was recorded in the control tank and in the tank with the 160 and 320mg/1 of CaCO3 respectively. The gills of exposed prawns were analyzed using photomicrography and the results showed that the organs had their normal morphological status after the exposure time, implying that CaCO3 is non-toxic to M. macrobrachion. The applications of the result to prawn culture and water quality management were discussed in details
