No information is available on siphonophores of the estuaries in India. The present investigation was taken up to study their occurrence and seasonal fluctuation in the Cochin Backwater, a tropical estuary. Data on salinity, temperature, zooplankton biomass and siphonophores were collected by analysing the samples obtained from 10 stations in the Cochin Backwater area between Alleppey light house (south of Cochin) and Marthomanagar (north of Cochin) during 1971 and 1972. Data reveal that siphonophores are present in the estuary, in the pre-monsoon and the post-monsoon periods except during, January. Analysis of data on rainfall, temperature and salinity shows that salinity has a more important role to play on the occurrence and seasonal fluctuations of the siphonophores.