The population structure and energetic burden of bopyrid isopod parasite Orthione griffenis on the eastern Pacific mud shrimp Upogebia pugettensis are estimated from size and weight relationships between parasite and host. U. pugettensis weight loss increases with O. griffenis weight but the high variance in the relation indicates that direct weight comparisons are insufficient to reveal most of the host-parasite energetic interactions. Environment, reproductive development, age, molt stage and the feeding histories of the hosts and parasites, not apparent from weight ratio comparisons, are likely to be important factors in their interaction. The high prevalence of O. griffenis among U. pugettensis and positive correlation between host weight loss and parasite weight nevertheless, indicate large impacts of these parasites on mud shrimp populations are occurring with corresponding effects on estuarine dynamics in Pacific Northwest estuaries.