The present study was conducted invitro to evaluate the acute toxicity of a broad spectrum neonicotinoid, imidacloprid used by farmers contently to control piercing-sucking pests in the paddy fields of Kuttanad, a part of the Vembanad wetland ecosystem which is a Ramsar site of international importance in the state of Kerala, India, to various life stages of Macrobrachium rosenbergii, a commercially important indigenous prawn of Kuttanad. The median tolerance limit (LC50) were calculated using the observations made on the mortality of prawns exposed to 5 different concentrations between the No Observable Effect Concentration (NOEC) and Lowest Observable Effect Concentration (LOEC) of biocides for a period of 96 h. LC50 values for 24, 48, 72 and 96 h of imidacloprid was determined by probit analysis using SPSS version 16.0. The study revealed that the 96 h LC50 concentrations for imidacloprid as 0.009, 5.0234 and 42.611 mg/l in post-larvae (0.041 ± 0.02 g), juvenile (3.5 ± 1.5 g) and adult (17.5 ± 2.5 g), respectively of M. rosenbergii. Based on the LC50 values, safe concentrations found out for imidacloprid using Hart’s formula were 0.002, 1.000 and 15.138 mg/l, in post-larvae, juvenile and adult M. rosenbergii, respectively. The field application concentration of imidacloprid is 0.003 mg/l which is 3 times higher than the safe concentration of imidacloprid in post-larvae, whereas it is lower than the safe concentration in juvenile and adult M. rosenbergii. The results revealed that the application of imidacloprid in the paddy fields of Kuttanad has a lethal effect on post-larvae of M. rosenbergii causing a threat to the existence of species in its homeland.