A new genus and species Bathione magnafolia of bopyrid infesting the deep-water galatheid crab Munidopsis depressa is described on the basis of material collected in the southern Gulf of California, in depths between 835 and 870 m and extreme hypoxic conditions (0.007 and 0.29 ml O₂/L). Females and males of the new species were compared with females and males of 21 closely related pseudionine genera. Infestation rates by these bopyrids of M. depressa were estimated from 1.2% to 2.5%. Pseudione humboldtensis Pardo, Guisado & Acuña, 1998, shares many characters with the new species and is transferred to the new genus.