The changes in volume of haemolymph of the horseshoe crab, Tachypleus gigas were more pronounced in females of size above 160 mm with maximum values in size group 241-260 mm. In males, the variations in the volume of haemolymph was not significant in sizes between 121-140 mm. An inverse relationship was observed between the volume of haemolymph and salinity of the environment. Maximum volume of haemolymph (57.3 ml) in females was observed when the salinity of the environment was low in October whereas, minimum (30 ml) in higher salinities during summer and post flood periods. The body weight of the crab was found to be affectEd. by the fluctuations in salinity. During flood period (October-November) average body weight of the crab increasEd. by 9.6% (size group 5), 6.4% (size group 6) and 9.8% (size group 7) in case of female, where as males showed only 3.09% higher body weight. During post flood period (December-January), the average body weight decreasEd. by 5.45; 7.2 and 18.2% in size group 5, 6 and 7 respectively in females and 6.1% in males. The variations in body weight with salinity were more pronounced in females than in males, during different seasons