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Some aspects on the biology of the mud shirmp.Upogebia dawsoni from the Requeson mangrove area, within bahia Concepcion, and Mulege river estuary, Mulege (Baja California sur, Mexico) are described.Collections of U.dawsoni were done in January (winter).March (spring) and July (summer) in 1986 at Requeson, and in October (fall) of 1985 and June (summer) of 1986, at Mulege.A total of 280 individuals were collected between 0.40 and 1.0 m in depth.Size, sex ratio, spawning period, egg number, stomach contents and external morphology were recorded.A seasonal comparative analysis between the two localities showed that the specimens from Requeson had greater lengths.In summer, females represented 6.67% and juveniles 93.33% while in fall, females represented 93.27%, males 2.58%, juveniles 2.25% and ovigerous females, stage II 0.90%, at Mulege.In winter, females comprised 90.90% and males 0.09%; in spring, females represented 61.11%, and males 38.88% during summer, ovigerous females stages I and II dominate, being 55.85% and 15.33% respectively, and males comprised 30.76%, at Requeson.Two spawning periods were detected, one in fall (Mulege), and other in summer (Requeson).The largest female (total length = 28.0 mm) carried out 2133 eggs and the shortest one (total length = 11.0.mm) had 119 eggs.Sediment, leaves and roots of mangrove insect wings, diatoms, dinoflageleates, bacteria, and sponge spicules were found in the stomachs