The omnipresent, free-floating perennial aquatic herb, water hyacinth is known to originate in the Amazon basin of South America. It has been distributed worldwide through travel and trades in different times of history. Though it was brought to India for ornamental purpose, due to higher rate of multiplication, it has become one of the major noxious aquatic weeds and spread almost all the water bodies in Indian sub-continent, including main rivers like Brahmaputra, Cauvery, and Ganga. Problems created by water hyacinth are associated with navigation, flood control, agriculture, irrigation, drainage and conservation of wild life. Water hyacinth weevils are the important bio- control agent of water hyacinth. First shipment of water hyacinth weevils were released in Assam during the year 1988. So,an experiment had designed to study the biology of these weevils and result of the experiment can provide a better idea about favourable environmental condition for these weevils to perform effectively to suppress the water hyacinth. The results obtained and the conclusions derived from this investigation are incorporated in this book.