Literature survey clearly indicates that fresh water prawns, despite being important member of food chain, good bio-indicator and having high economical value, have not been properly studied in relation to metal toxicity. Considering economical and good bio-indicator value of fresh water prawns, present work has been taken into account to evaluate behavioural, haematological and histopathological effects of mercuric chloride and nickel chloride on Macrobrachium lamarrei and Macrobrachium dayanum, prawn species, abundantly available in Lucknow. The research work incorporated in present thesis is represented in three sections. Section-I deals with bioassay experiments; Section-II deals with behavioural, haematological and histopathological effects of mercuric chloride while Section-III deals with behavioural haematological and histopathological effects of nickel chloride on fresh water prawns, Macrobrachium lamarrei and Macrobrachium dayanum, good bio-indicators and potential animals for fresh water aquaculture.