Data obtained on numerous cruises, from southern New England to the Caribbean Sea, between September 1956 and August 1963 form the basis for this study on the relationship of flora to hydrography. Seaward from the southern New England coast, with salinity increasing, the quantity of phytoplankton in near-surface water (0-25 m) decreases while the proportion of oceanic species increases. In the Sargasso Sea, in summer, the stratification and flora are similar from north to south, but in winter, when there is a difference in stratifi cation of the upper 200 m in the northern and southern portions, there is a parallel difference in the abundance of many species....