Solen cylindraceus (Hanley, 1843) is an infaunal bivalve that in the St Lucia Estuary is currently restricted to the southern part of its South Lake, having disappeared from the northern reaches due to persisting hypersaline conditions (>70 ‰) and air exposure at low water levels. The system experiences marked fluctuations in salinity due to quasi-decadal changes from wet to dry periods. In this study, the salinity tolerance of S. cylindraceus is determined using both shock and gradual change tests. Animals were collected at Catalina Bay (eastern shores of South Lake) and acclimated under laboratory conditions to naturally occurring salinities of 50 ‰ and 45 ‰ for the shock and gradual test, respectively. Mortalities were recorded for animals exposed to a sudden change in salinity, using eight different treatments ranging from 0 to 80 ‰. The second test involved exposing bivalves to a gradual change in salinity, using eight different treatments from 0 to 85 ‰. In the shock test, the lower salinity tolerance limit for S. cylindraceus was 30 ‰ and the upper 60 ‰, while in the gradual test, these limits were 15 and 65 ‰, respectively. The time it took for 50 % of animals to die increased from the shock to the gradual test for 10, 20 and 70 ‰, and decreased for 0 and 80 ‰. This knowledge may be useful towards predicting major crises in the S. cylindraceus populations, as drought and flood events alternate in the region. Major losses will be expected when salinities exceed 65 ‰ during dry phases or drop below 15 ‰ during flood events.