Survival of the estuarine prawn Palaemon longirostris (Milne Edwards) was measured at various salinities (0.5, 3.5, 7, 15, 22, 35 and 43) and temperatures (4, 12 and 20ºC). Individuals, collected in summer and winter, were sorted based on carapace length and presence of eggs into "small" (10-18 mm), "medium" (18-24 mm), "large" (> 24 mm) and ovigerous (> 24 mm) groupings, and monitored over 7 days in constant conditions. Summer prawns were very euryhaline and generally had > 90 % survival at each salinity, however, significant effects of salinity, temperature and prawn size on survival were isolated (ANOVA, P < 0.005). In general, survival was reduced in hypersaline conditions (S, 43), particularly for small prawns at 4ºC. During winter, prawns were less available than summer, and only medium and large groupings were studied. Winter individuals were also very euryhaline but had significantly lower survival than medium and large summer prawns (ANOVA, P < 0.001). Interestingly, salinity and temperature did not have any significant influence on the survival of winter prawns, but medium prawns were less euryhaline than large individuals (ANOVA, P < 0.005). The data help explain the presence throughout the year of all post-larval stages of P. longirostris in the dilute saline regions of estuaries where salinity fluctuations are pronounced and rapid.