The effects of tributyltin fluoride in the sea water on the Hydroides ezoensis have been studied. The observation with light and electron microscopies showed that the toxicant diffused from the body surface through the blood sinus into the brain neuroganglion could lead to serious swelling of the central nervous system and set off a chain pathological reaction. There may be two possibilities about the toxic mechanism of tributyltin fluoride. One is the change of the membrane permeability, the other is the inhibition of oxidative phosphorylation, but the former is more strongly suggested.