1.78% of Nematopalaemon tenuipes Henderson were found to be infected by the branchial isopod parasite Bopyrus squillarum Latreille at Bombay. Both the sexes of N. tenuipes were equally infected and they showed two peaks of infection - May and October, in all the three years from 1979 to 1981. The peaks of infection were observed to coincide with the peak recruitment of N. tenuipes. The infected individuals were small, mostly below 50 mm in length. The size of the host and its parasite also showed a positive correlation. The parasite affected the growth by 34% and made the host stunted and weak in appearance which was reflected in their length-weight relationship showing significant differences. Death appears to be caused by the slow attrition caused by the blocking of the branchial surface by the parasite