We report the observation of a "superquenching" for water-soluble poly-l-lysine derivatives containing an appended cationic cyanine dye on each repeat unit. The formally nonconjugated cyanine dye chromophores strongly associate in a "J" aggregate structure characterized by a sharp red-shifted absorption (compared to the monomer) and a similarly sharp red-shifted fluorescence. Superquenching is manifested by very large "Stern-Volmer" constants for fluorescence quenching by oppositely charged electron accepters or energy transfer dyes; substantial quenching is observed at levels of quencher corresponding to one to four molecules per polymer chain. The quenching observed for these polymers is equivalent or greater to that previously observed for conjugated polyelectrolytes. We have been able to exploit the superquenching of the "J" aggregate polymer fluorescence in a competitive bioassay.