The impact of photoperiodism on the ovarian maturation and gonadosomatic index of fresh water crab, Paratelphusa hydrodromous (Henderson, 1983) was investigated in the laboratory over a period of two weeks. 20 healthy female crabs of uniform weight and size were collected. The experimental crabs were divided into three groups namely, control, continuous dark and continuous light. The ovarian maturation was investigated by microscopic and histological observations. G.S. I (ovarian indices) of all the 3 groups has been calculated. There was a considerable increase in size of gonads and total egg production over a very short period in continuous dark group of test animals, on the other hand control animals gained maturity after a long period. The GSI revealed after the study shows marked difference as that of control group. Therefore, it was concluded that complete darkness is useful to accelerate ovarian maturation in Paratelphusa hydrodromous.