RECENT investigations of the sensitivity of certain groups of fishes to minute local potential changes have revealed the fact that all members of these groups possess specialized neuromasts in addition to their ordinary lateral-line organs. Whereas the ordinary organs are used as ‘distant touch’ receptors, responding to mechanical stimuli, several facts and considerations have led to the assumption that the specialized or ‘ampullary’ organs are used as electroreceptors1. The ampullae of Lorenzini, which occur on the head of all elasmobranchs, are typical and important representatives of this latter group of derived lateral sense organs. Electrophysiologically, the ampullae have been shown to respond to slight potential changes (1 µV/cm)2. On the other hand, behaviour experiments revealed the occurrence of muscular reactions in rays and dogfishes in response to equally minute or even weaker electrical stimuli (0.1 µV/cm)3.