Thymic organogenesis and T-cell lymphopoiesis are crucial interdependent pr
ocesses that establish a functional vertebrate immune system. The current u
nderstanding of vertebrate thymic development during embryogenesis remains
incomplete and would benefit from novel approaches. The zebrafish Danio rer
io is a powerful developmental and genetic system for the dissection of ear
ly events in the ontogeny of the immune system. Forward genetic screens hav
e uncovered genes involved in hematopoiesis, and specific screens are being
designed to examine the genes that regulate T-cell development and the ori
gin of the thymus. Studies of the zebrafish should improve our understandin
g of lymphoid development in vertebrates.