The effect of formalin on the organoleptic parameters, biochemical composition and storage quality of iced and frozen stored giant freshwater prawns (Macrobrachium rosenbergii) was investigated. The organoleptic characters such as appearance, textural condition, colour, odour of both formalin added and non-added prawns were evaluated. The quality of non-added prawns was judged good up to first 3 days of iced storage and remained acceptable in terms of commercial standard for processing in the industry up to 6 days. On the other hand, formalin added samples were organoleptically acceptable up to 8 days. The expressible moisture content of both formalin added and non-added prawns continued to increase gradually during 90 days of frozen storage and it was higher in non-added sample. Study of proximate composition at initial storage showed that there were variations in composition between formalin added and non-added samples. At the end of 10 days of iced storage, there was little change in composition in both the cases. To assess the freshness of the prawn samples, TVB-N value and myofibrillar protein solubility were determined. Under iced storage, there was a clear trend of increasing TVB-N value both in formalin added and nonadded samples with higher value in non-added sample. The initial myofibrillar protein solubility of formalin added prawn was 88% and that of non-added sample was 80%. After 10 days of iced storage, the protein solubility of both the samples decreased to 50% of their initial value. The solubility of myofibrillar protein of formalin added prawn was lower than non-added sample. Overall loss of freshness took place much faster in non-added prawn sample than the formalin added sample both under iced and frozen storage conditions.