The biology of Lemaea cyprinacea (L. cyprinacea) parasitizing carp (Cyprinus carpio) was studied at Abbassa Fish Hatchery (AFH) and Central Laboratory of Aquaculture Research, Abbassa (CLAR) during 2000 —2001. The life cycle, morphometric characteristics and host para­site relationship of this species were described in this study. All develop­mental stages of L. cyprinacea life cycle in vitro under different temper­atures were distinguished by the reflexed claws on its antenna. The development of the antennually segmentation and setation pattern are traced from third copepodids to adults of both sexes. Filling the fish ponds with water five days before nursing the fry or fingerlings of carp (Cyprinus carpio) Is sufficient to break the life-cycle and prevent the in­fection.