The quantitative geographical distribution of euphausiids of the tropical Indian Ocean has not been studied in detail. Considering this lacuna in our knowledge, a study was undertaken on the spatial distribution of euphausiids of the southeastern Arabian Sea (west coast of India including the Lakshadweep Sea) for the first time. The Arabian Sea and the Bay of Bengal, closed at their northern parts, present special hydrographical features which influence the distribution and abundance of zooplankton in these areas. Here the IO'N latitude is thought to be an effective barrier against the penetration of several oceanic species of euphausiids northwards. This is because of the significant changes in the water quality north of 10°N, being influenced by the discharge from major rivers of the Indian sub-continent. During the present study the spatial distribution of 22 species of euphausiids has been worked out as a result of which it has been found that the species namely Thysanopoda monacantha, T. tricuspidata and Stylocheiron maximum which were believed hitherto being restricted to areas south of 10 N are distributed further northwards.