The feeding-deterrent properties of a suite of common holoplankton (nine species representing five phyla) collected from oceanic waters near Bermuda were investigated. The common planktivorous fish Abudefduf saxatilus (sergeant major) was used as a model predator. With the exception of the salp Pegea bicaudata, all plankton had significant ichthyodeterrent properties compared to squid tissue controls. These plankton generally lack structural defenses and are passive or sluggish swimmers; our observations suggest that they may derive protection by harboring defensive chemistry. Chemical defenses would have important implications for the regulation of material and energy flux in oceanic food webs, and such defenses may help explain the abundance of coexisting species, all competing for similar resources, in a seemingly homogeneous habitat (i.e. the paradox of the plankton).