Aporobopyrus bourdonis, new species, earlier reported from the Pacific coast of Mexico but never described, infests Petrolisthes edwardsii (de Saussure), the same host species as before, in Costa Rica. Aporobopyrus muguensis Shiino, 1964, previously reported several times along the coast of California infesting Pachycheles rudis Stimpson and P. pubescens Holmes, is redescribed on the basis of material from Monterey, California, infesting P. pubescens. Aporobopyrus trilobatus (Nierstrasz and Brender à Brandis, 1925), previously known from the Caribbean Sea and the Pacific coast of Mexico, infests Petrolisthes ortmanni Nobili, a new host record, in Costa Rica. Discomorphus magnifoliatus, new genus, new species, infests Petrolisthes cinctipes (Randall) in central California. Synonymies are given for all species. Included is a summary of all porcellanids known to bear bopyrid parasites in the eastern Pacific.