The mud shrimp Upogebia pugettensis is native to intertidal areas of Pacific estuaries, and is viewed as a pest by the shellfish industry because of its ability to disturb sediment and bury oysters.   It is estimated that 50-90% of Eastern Pacific reproductive sized Upogebia are infested by the parasitic bopyrid isopod, Orthione griffenis.  Orthione appears to castrate and prevent the reproduction of its host, and could potentially be used to biologically control Upogebia populations.  Some parasitic castrators feminize the external sex characteristics of their male hosts resulting in a reduced size and feminine appearance.  We looked for evidence of feminization by measuring carapace and chela lengths of infested and uninfested Upogebia.  We found significant differences in chela sizes of Upogebia infested with isopods of varying brooding stages.  Infested female Upogebia had larger chela than uninfested females of the same size.  Conversely, infested male Upogebia had a reduced chela size compared to uninfested males of the same size.  This result indicates that feminization does occur among infested male Upogebia in the form of a reduced, and more feminine, chela.   Further studies are needed to understand the physiology of feminization and determine if it is due to hormonal disruption or simply nutrient loss.