A coastal lagoon, which was open to theocean during the first 6 months of a period of observation and sampling, was closed off during the following 8 months of study. Salinity increased from 35 o/oo to 60 o/oo. Compared to conditions present while lagoon was open to the ocean, after the closing of the bar summer water temperatures were 4-5[degree]C. higher, the pH was generally 0.6 to 1.0 pH unit higher, and concentrations of dissolved oxygen were slightly lower. Since both pH and dissolved oxygen had been as variable prior to closing of the lagoon as afterward, the main physical factors of concern were temperature and salinity, of which the latter showed the greater change. Of 10 species of fishes trapped in the lagoon when it closed, the majority tolerated the changing conditions until the salinity reached 50-55 o/oo. Hysopsetta guttulata withstood the conditions a little longer - until the salinity was 55/60 o/oo. No upper limit of tolerance was observed for Fundulus parvi-pinnis and Girella nigricans, or for Atherinops affinis littoralis, some which were still thriving, at a salinity of 63 o/oo, when the study terminated.