This study aimed to estimate the cations (calcium, potassium & sodium) in the fresh water field crab Paratelphusa hydrodromous. The sodium concentration in hepatopancreas gradually decreased are size of the crab increase and there was an inverse relationship between gonad and hepatopancreas in sodium ion concentration. In muscle and hepatopancreas the calcium ion concentration gradually decreased as size group advanced (increase). However in gonad it varied in different size groups and the variation in the calcium ion concentration is may be correlated with moulting cycle of the crab. The potassium ion concentration of P. hydroromous in selected tissues showed slight variation in different size group. However in gonad, hepatopancreas an inverse relationship was noticed.