Darwin recognized the potential significance of animal-assisted dispersal for the geographic distribution of freshwater species. Phoretic interactions are assumed to contribute to the secondary (post-establishment) spread of invasive freshwater invertebrates, but vertebrate animals reported to disperse invasive bivalves are limited to amphibians and waterfowl. We present evidence of a novel phoretic interaction between the zebra mussel (Dreissena polymorpha) and a freshwater cyprinid minnow, the lake chub (Couesius plumbeus). To our knowledge, this is the first field-documented case of phoresis involving a freshwater bivalve and a fish. We suggest that this interaction will exacerbate risks of within-basin spread of zebra mussels via fish migration and overland transport of mussels by anglers carrying baitfish from invaded waterbodies.