The temporal variation in the abundance and species composition of penaeid shrimp postlarvae and juveniles was studied in a tidal channel of the Colorado River estuary. Biweekly sampling during flood and ebb tide was conducted from March through November 2000. Postlarvae of the blue shrimp, Litopenaeus stylirostris, were observed beginning in May, with highest densities in early June during flood tide. A much smaller peak in densities occurred in late September, with few postlarvae found in October and November. Juvenile L. stylirostris were present in highest densities during ebb tide in mid-June, with smaller peaks in mid-July and late September. Densities of postlarval and juvenile brown shrimp, Farfantepenaeus californiensis, were much lower than those of L. stylirostris and without clearly defined peaks. The growth rate of juvenile L. stylirostris appeared similar to rates (30–60 mm month–1) reported for penaeid shrimp from the Pacific Ocean, Gulf of Mexico, and Australia. Management decisions affecting the Upper Gulf of California and Colorado River Delta Biosphere Reserve need to be cognizant of the importance of May and June as critical months in the use of the estuarine tidal channels by these commercially important species.