We model the economics of open ocean grow-out of two species of shellfish - sea scallops (Placopecten magellanicus) and blue mussels (Mytilus edulis). Models are informed by the results of offshore grow-out experiments conducted in southern New England in the late 1990s, and illustrate conditions under which open ocean shellfish grow-out operations can be profitable. We examine the economic viability of four alternative approaches to open ocean scallop farming: seabed seeding and three variations of cage culture (lantern cages, bottom cage trawls, and bottom cage clusters). For each alternative, we estimate capital and operating costs and revenues over a 20 year period. We assume a two-year cycle from collection of juveniles to harvest, and scale the farming operation in every case to produce 100,000 pounds of scallop meat per two-year cycle (that is, every other year). Under baseline assumptions, the only profitable alternative is seabed seeding. A 100,000 lbs/cycle seabed seeding operation requires less than $400,000 in start-up capital and pays back the initial investment in four years. Seabed seeding requires a lease area of about 150 acres and use of a large scallop vessel about 3 months out of the year, on average. Cage operations are not profitable because higher survival rate and growth are not enough to justify added cost of buying, maintaining, deploying, and harvesting the cages and associated moorings. Although they require smaller lease areas, cage operations demand between $1 million and $2 million in startup funding. Of the three alternatives, bottom cage trawls come closest to break-even because gear costs are relatively modest.