Properties of electroreceptors in the lateral line organ of three different species of fresh water, weakly electric fishes (Hypopomus and Gymnotus) were studied by recording single fiber responses from the lateral line nerve. The fiber responds to each pulse of the electric organ of the same fish with a short train of one to a dozen or so impulses at 200-500/sec. A small conductive object right over the receptor decreases the number of impulses in a train, whereas just in front of or behind the receptor it increases the number. An imposed pulse of electric current in the water which produces a train of impulses at its onset will produce a train at its cessation when the polarity of the pulse is reversed. The threshold for impulse train is 5-30 mV/cm in the water near the surface of the fish, and an additional impulse in each train is produced by an increase of 1-10% (50-1,000 [mu]V) in pulse voltage. Increasing the duration of an imposed pulse to 3-4 msec causes an increase in number of impulses in a train, but further increase in duration adds no more to a train; i. e., the integration time constant of the receptor is smaller than 3-4 msec.