Electrolocation behavior of Apteronotus leptorhynchus was studied by monitoring the animal's ability to maintain orientation to a variety of moving electrolocation targets. The primary goal of this study was to determine the relative effectiveness of various types of electrical 'jamming signals' in disrupting electrolocation performance. 1. Two measures of electrolocation performance were used: The latency between the electrolocation target motion and the fish's following response, and the minimum distance separating the fish from the target during the target movement sequence. Latency increased and minimum fish-target distance decreased as target size was decreased, and when large diameter ceramic targets were used as control stimuli the fish were less able to avoid, and frequently collided with, these 'electrically transparent' objects. 2. Four types of jamming signals were used in attempts to mask the electrosensory input used for electrolocation. Broad-band noise and sinusoidal signals, different in frequency by a few Hz from the animal's personal electric organ discharge (DF stimuli), were used to jam the tuberous electroreceptors. Five Hz and 50 Hz sinusoidal signals were used to jam the low-frequency or ampullary receptor system. Both the noise and the DF stimuli were effective in reducing electrolocation performance, and the threshold intensity for behavior deterioration was about three-fold lower for DF stimuli as compared to the noise. The rate of change of response deterioration as a function of increasing jamming intensity was, however, not different for these two types of stimuli. Neither the 50 Hz nor the 5 Hz jamming signals caused electrolocation deterioration when presented alone. However, 5 Hz jamming, when added to either the noise or DF jamming, did result in significant increments in response deterioration. This suggests that the ampullary receptors can provide supplementary information for electrolocation. 3. Electrolocation performance deterioration was also studied with various difference frequencies between an animal's EOD and the sinusoidal jamming stimulus. Increasing DF results in decreased electrolocation deterioration, but even at the highest DF frequencies used (128 Hz) significant response degradation was observed. 4. The apparent differences in the effectiveness of noise and DF stimulation in reducing electrolocation performance are largely accounted for by the differential effects of the tuberous electroreceptor filter characteristics on these two types of signals.