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The qualitative and quantitative distribution and seasonal variation of luminous bacteria in the sediments of Vellar estuary, Porto Novo, South India (11° 29'N, 79° 46'E) was studied. The luminous bacterial population ranged from 5.1 x 103 to 3.2 x 104 CFU/g dry weight in station I and 2.4 x 103 to 9.1 x 103 CFU/g dry weight in station II. The salinity of the estuarine water appeared to have an indirect relationship with luminous bacterial counts in the sediment. The maximum number of luminous bacteria were recorded during summer followed by a decline before the monsoon. The counts were at a low level during monsoon and then started to increase after the monsoon season. Vibrio harveyi, V. fischeri and Photobacterium leiognathi were the species recorded in the sediments. V. harveyi was the dominant species being present throughout the year.Luminous bacteria are widely distributed in the marine environment, from surface to deep water and from the equator to the polar regions. They are ecologically versatile, present in water, sediment and also harboured in the light organs of some fish and gut of many marine organisms (7). Though the association of these bacteria with water and marine animals has been well studied, not much information is available on their densities and distribution in sediments. Hence the present study has been undertaken to elucidate the qualitative and quantitative distribution and the seasonal variation of luminous bacteria in tropical estuarine sediments. MATERIALS AND METHODSSediment samples were collected for a period of one year (from April, 1981 to March, 1982 from two stations in the Vellar estuary located at Porto Novo (11 0

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