1. Material discharged by isolated nematocysts of Metridium senile senile Linnaeus (1767) has been found to be toxic to crayfish Orconectes virilis Hagen (1870), O. propinquus Girard (1852), and Procambarus econfinae Hobbs (1942).2. The toxin appears to be composed of a proteinaceous, undialyzable fraction, and a dialyzable fraction containing two aromatic amines.3. The amines are probably fused or polysubstituted ring compounds, one of which is carboxylated.4. The crude toxin has varying potency, depending upon the species of assay animal used.5. With the possible exception of histamine, none of the compounds previously suggested as possible components of the nematocyst toxin were found.