We are currently engaged in the development of methodologies for gene transfer to crustaceans, using Macro brachium rosenbergii as a model system. Because this species retains its eggs during embryogenesis, it was necessary to develop a means to bring fertilized eggs to hatching in vitro. This permits us to remove and manipulate eggs, and to introduce foreign DNA at any desired stage of development. The protocol we use simulates the natural environment to which the eggs are exposed during their period on the abdomen: under normal circumstances they are incubated in a continuous flow of water "fanned" by the female's swimmerets. Not only does this provide oxygenation, but it presumably protects them against fungal and bacterial infection. Infection usually kills removed eggs rapidly. By substituting carefully controlled agitation, constant aeration, and timed changes of water, coupled with antibiotic and antifungal agents, we are able to detach eggs from the female as early as 16 hours after fertilization, and hold them in a flask to the stage of free swimming larvae. Hatching is completed in the normal period of 21 days, entirely in the artificial system. Larvae produced under these circumstances are morphologically indistinguishable from those hatched out in vivo and exhibit normal behavior. This system provides apparently-normal embryos at any desired stage for experiments in genetic manipulation. So far as we are aware, this is the first demonstration of the practicality of an in vitro hatching system for any commercially important decapod crustacean, and will be of great value in future programs for genetic engineering and/or selective breeding of these animals.