The Snake River Pilose Crayfish, Pacifastacus connectens (Faxon 1914), previously known only from the middle Snake River drainage in south-central Idaho west to the Harney Basin in southeastern Oregon, was discovered in the lower Deschutes River in north-central Oregon co-occurring with Signal Crayfish, Pacifastacus leniusculus (Dana 1852). Two groups of obligate ectosymbionts (i.e., entocytherid ostracods and branchiobdellidans) associated with these 2 Pacifastacus species displayed asymmetrical patterns of host switching, with host exchange primarily occurring from P. leniusculus onto P. connectens. Our results have implications for understanding patterns of host specificity over short ecological timescales but also raise questions about the persistence of these patterns over time.