After having collected the plankton samples in the Java Sea on which I have reported in Treubia XVII, 1939, I thought it desirable to gather similar samples from a more oceanic area of the East Indian seas, in order to be able to make a comparison between the two collections and to try to find out the characteristic differences between the two regions. It seemed to me interesting to choose in the first place Sunda Strait for this purpose, as it connects the shallow Java Sea with the deep Indian Ocean and, therefore, might offer all degrees of transition from the plankton of the former to that of the latter. The next year, then, in 1933, I made a cruise in Sunda Strait in the months of April-May, the transition months between the (wet) west monsoon and the (dry) east monsoon. The stations visited may be seen from the accompanying chart. The depth increases considerably in the direction Java Sea —> Indian Ocean but is everywhere sufficient to allow the making of vertical hauls with the plankton net from 50 meters depth to the surface. For the sake of convenience, therefore, all the hauls in Sunda Strait have been made in this way. This was not possible the year before as at several stations in the Java Sea the depth is insufficient. The same net was used as the foregoing year: width of the mouth 1 1/3 m, length 4 m, Swiss plankton gauze nr. 3. In the Sailor's Guide for the East-Indian Archipelago we read : ""This diurnal tidal stream (seil, in the Java Sea) is weakened towards the NE and strengthened towards the SW in the first place by a current to the SW starting from Bangka-Strait, which runs along the SE-coast of Sumatra and through Sunda Strait at the rate of more than 0.5 Mile per hour.