Salinity and salinity changes were studied over the range of the polychaete Neanthes lighti in the estuary of the Salinas River, California, during the yrs. 1948-1952. Not only are mean salinities different in various parts of the range of N. lighti, but each locality has a characteristic annual pattern of salinity variation depending upon fresh water discharge into the estuary. The pattern of salinity variation is traced through years of low and moderate rainfall, and its upset during the flood season of 1951-52 is reported. Worms inhabiting marshy areas near the river mouth are exposed to variable but high salinities (40 to over 100% of sea water); those in the upper reaches endure uniformly oligohaline conditions (2-3% of sea water) for most of the year. Either of these areas is more favorable than the lower reaches of the estuary that receive full tidal exchanges for part of each year. Over its range. N. lighti is associated with such typically brackish water spp. as Neosphaeroma oregonensis, Anisogammarus confervicolus, and Corophium spinicorne. Its range only slightly overlaps the ranges of such fresh water forms as Dugesia sp., leeches, or odonata larvae. Corophium stimpsoni is found in water fresher than that inhabited by N. lighti or the crustaceans associated with it. The populations of N. lighti of the upper reaches are ecologically and reproductively isolated from those of the more saline marsh channels, and some evidence for the existence of physiological distinctness between these populations were found.