The optimum salinity for the development of eggs of Long Island Sound clams (Venus mercenaria) was about 27.5 parts per thousand. No normal larvae developed at salinities of 17.5 ppt or lower. The upper salinity limit for development of clam eggs appeared to be 35.0 ppt and only an occasional normal larva developed at this salinity. A salinity of 26.0 to 27.5 ppt, or higher, was also optimum for growth of clam larvae, with 20.0 ppt the lower limit for practical culture work. Clam larvae grew reasonably well at a salinity of 17.5 ppt and many reached metamorphosis; however, they were so weakened and sluggish that they died during, or immediately after metamorphosis. At 15.0 ppt none of the larvae reached metamorphosis although some lived for 10 days or more and showed appreciable growth. At 12.5 ppt straight-hinge clam larvae showed no growth and all were dead by the 8th or 10th day. Optimum salinity for development of fertilized eggs of oysters that had developed gonads at 26.0 to 27.0 ppt, was 22.5 ppt, although some normal larvae developed in salinities as low as 12.5 ppt and as high as 35.0 ppt. At salinities below 22.5 ppt the percentage of eggs that developed to the straight-hinge stage dropped off until at 15.0 ppt only 50 to 60% developed normally. At 12.5 ppt almost none of the eggs developed into normal straight-hinge larvae. In other experiments Maryland oysters that had developed gonads in their native habitat (salinity 8.7 ppt at the time of collection) were spawned at salinities of 7.5, 10.0 and 15.0 ppt. Under these conditions some oyster eggs developed into normal larvae at 10.0 ppt and at 7.5 ppt larvae only slightly smaller than normal were produced. In fact 10.0 to 15.0 ppt was the optimum salinity for development of these eggs and 22.5 ppt was the upper salinity limit. How well these larvae would grow and whether they would be of any value in obtaining a set has not yet been determined. When oysters that had developed gonads at a salinity of 26.0 to 27.0 ppt were used as parents, optimum salinity for growth of larvae after reaching the straight-hinge stage was 17.5 ppt. Good growth was obtained at a salinity of 17.5 ppt, but at 15.0 ppt the rate of growth of larvae was appreciably slower, although they did grow to setting size, and set. At 10.0 ppt larvae grew very slowly and it is doubtful that any would reach setting stage at this salinity. The older the larvae, however, the better they withstood a salinity of 10.0 ppt. Larvae that were reared almost to setting stage at a salinity of 26.0 to 27.5 ppt continued to grow and set when placed in a salinity of 10.0 ppt.