Larval supply and early postsettlement mortality are crucial to the maintenance of most manne populat~ons Larval supply may be espec~ally important in regulating populat~ons of blue crabs Callinectes sapidus because many larvae could be lost during long migrations between estuanes and offshore waters We examined the relative contnbutions of different~al transport and habltat preferences of megalopae (postlarvae) to 5 potential nursery habitats In Mobile Bay and M~ssissippl Sound Alabama USA Settlement In each habitat was determined dally during summer and fall for 2 yr and was related to sea surface temperature wind stress, tides, current velocity and the lunar and tidal amplitude cycles In 1990, most megalopae settled when winds blew megalopae onshore and tidal amplitudes were m~n i m a l In 1991, onshore winds were light and Infrequent, and megalopae settled even more densely d u n n g minlmum amplitude tides Sem~dally collect~ons of megalopae at dusk and dawn revealed that most megalopae recruit to estuanes during nocturnal flood tldes, which only occur dunng summer and fall in this diurnal tidal regime Thus, onshore winds facilitated transport of megalopae into estuanes ep~sodically, but megalopae recruited regularly and abundantly d u n n g nocturnal mlnilnum a m p l~t u d e flood t~d e s even when onshore w~n d s were light or absent most megalopae were collected in the lower bay although some megalopae settled at the head of the estuary 50 km away from the baymouth Megalopae required about 2 d to travel this distance as they became increasingly conlpetent to settle from the plankton Physiological tolerances may e x p l a~n why most larvae settled in high salinity waters, but differences in current regunes may best explain differences in settlement among sites In the lower bay Several types of substrate were transplanted from nursery habitats to a sandy area at the baymouth to d e t e r m~n e whether megdlopae prefer to settle in seagrass beds, marshes or unvegetated substrate once they arnve at a site Megalopae strongly preferred to settle on vegetation, but ~t remains unclear whether or not they discriminate among plant types Thus, passlve dehvery and habitat preferences of megalopae both may d e t e r m~n e the initial distnbutions of l u v e n~l e blue crabs