The effects of Extracellular products (ECP) of Aeromonas hydrophila on Penaeus indicus were assessed by haemolymph study. ECP was obtained by growing the pathogenic bacterium on sterile cellophane sheets over TSA plates followed by suspension in normal saline, centrifugation and filtration. Protein profile of ECP was analysed by SDSPAGE. Five groups of P, indicus and a control group were maintained to study the effect of five different concentrations of the ECP of A. hydrophila on haemolymph factors. There were rapid behavioural changes and gross pathological alterations like loss of appetite, abnormal swimming behaviour, melanisation on abdomen and soft shell syndrome. The mortality rate for each concentration of ECP was determined. The total haemocyte count was assayed in all groups at different hours. There was reduction in the number of haemocytes with each higher concentration of the ECP. These changes are attributed to the virulent factors in the ECP of Aeromonas hydrophila.