Salinity tolerance was compared between an inland population of red swamp crawfish, Procambarus clarki, from Baton Rouge, Louisiana, and a coastal marsh population from Grand Chenier, Louisiana. Newly hatched crawfish from each population were killed in less than one week in salinities of 15, 20 and 30 ppt. Crawfish, 30 mm in total length, withstood salinities up to 20 ppt, but died in 30 ppt in two to three days. Crawfish, 40 to 120 mm in total length, showed no significant mortality after one week in salinities up to 30 ppt. Thirty-millimeter crawfish exposed to salinities of 0, 10, 20 and 30 ppt for four weeks grew very little when fed fresh fish flesh, tropical fish food pellets, and Oedogonium sp. All 30-mm crawfish in 30 ppt died. Growth varied inversely with salinity. Forty- to fifty-mm crawfish held in 0, 10, and 20 ppt salinity for four weeks had average increases in weight of 4.4, 13.5 and 4.9%, respectively. They were fed mixed green algae, which they ate continually. Growth in 10 ppt was significantly greater <.05) than that in 0 or 20 ppt.