Water hyacinth is an aquatic plant coined to multifarious activities including its role as an obnoxious weed with tremendous economic and aesthetic implications. Parthenium (Parthenium hysterophorus L.) is a terrestrial weed often put in the category of world’s worst weeds now assuming status of India’s national weed affecting human health, agriculture, environment and natural biodiversity with tremendous economic implications. The weed has toxic and phytotoxic constituents comprising of phenolics and terpenoids- two major chemical classes implicated in toxic and allelopathic interactions of the species. The species has wide range of constituents in its plant parts, but a few allelochemicals have been investigated for water hyacinth control. Many phenolic acids have been investigated for inhibitory activity on water hyacinth, of which p-hydroxybenzoic acid appeared to be of potential herbicidal activity at 100 ppm, a lethal level for the aquatic weed. Major sesquiterpene lactone parthenin is another allelochemical which has been shown to be a potential herbicidal for water hyacinth at 100 ppm, which killed the weed irrecoverably.  Investigations undertaken on the aspect of control of water hyacinth by Parthenium allelochemicals showed that allelochemical crude and constituent allelochemicals and other secondary metabolites from Parthenium leaf herbicidal for the water hyacinth. The allelochemicals could be used under certain situations for managing a weed (water hyacinth) by another weed (Parthenium) fostering a concept of weed against weed. The little work undertaken so far on screening of Parthenium constituents including allelochemicals for herbicidal activity on water hyacinth and other aquatic weeds pointer to necessity of taking up the investigations on these lines intensively, which might facilitate the development of natural herbicides and their formulations and provide lead for the development of newer synthetic herbicides with novel chemistry for more effective and environment friendly management of water hyacinth and other aquatic weeds.