Species composition, abundance and distribution of hydromedusae from Dharamtar estuarine system, adjoining Bombay Harbour, Maharashtra, India were investigated during 1984-1985. Twenty six species belonging to 19 genera were obtained from this area. Maximum species diversity was observed at st. 1 (coastal zone). A downward trend in species diversity was discernible towards the upper reaches of the estuary. Highest population density of hydromedusae was observed at st 4 (inner zone) and it was due to the swarm of Ostroumovia inkermanica, 6450/100m sup(3) during September. The swarms were composed of chiefly juveniles. The more number of species of the outer zone was due to the occurrence of holoplanktonic species. O. inkermanica, Phialidium brunescens, Euphysora bigelowi and Blackfordia virginica were recorded from all stations at least in a few months of the year. A marked species succession was discernible at some stations while species co-existence also was observed at other stations. Postmonsoon season (October-January) and late monsoon season (September) were found to be favourable for the growth and activity of hydromedusae in this estuary.