1. The effects of tetanus toxin have been investigated both on central nervous and peripheral neuromuscular systems of goldfish.2. Tetanus toxin kills goldfish when administered in minute doses. The lethal effect is temperature dependent. Unlike mammals, in which tetanus toxin produces spastic paralysis and convulsions, the tetanus‐intoxicated goldfish die with an apparently flaccid paralysis.3. Inhibition and excitation were investigated on the Mauthner cells of goldfish which had been paralysed with tetanus toxin for at least 24 hr, and which were subsequently kept alive for up to 3 days by perfusing the gills with oxygenated water. In such fish, antidromically and orthodromically excited Mauthner cells were quite normal, and there was no apparent effect on either the collateral inhibition or the crossed VIIIth nerve inhibition.4. Local injection of sublethal doses of tetanus toxin into the pectoral fin muscles produced local paralysis of the fin. Nerve—muscle preparations were isolated from such goldfish; in tetanus toxin‐paralysed fins, the muscle no longer responded to stimulation of its nerve. The nerve compound action potential was still present and the muscle still responded vigorously to direct electrical stimulation.5. It is concluded that the major part of the lethal action of the toxin in the fish must be ascribed to a peripheral effect, the blocking of neuromuscular transmission. The inhibitory neuronal systems acting on the Mauthner cells of the goldfish, in apparent contrast to those acting on mammalian spinal neurones, are highly insensitive to tetanus toxin.