The copper and protein contents have been determined in the blood and also in the purified haemocyanin of the freshwater field crab,Oziotelphusa senex senex (Fabricius) in relation to sex. The haemocyanin content was fairly higher in the blood of female crabs when compared to the males. However, the copper-protein ratio in the purified haemocyanin of both male and female crabs is 0·167%. Based on the copper content, the minimal molecular weight of the functional unit of theO. senex senex haemocyanin has been calculated as 76,120 daltons. The sex-based difference in the haemocyanin concentration has been discussed in the light of the existing literature.