The jamming avoidance response (JAR) in young weakly electric fish, Eigenmannia, develops at the onset of a functional electrosensory phase-coding system, a neural pathway that is critical for the performance of the JAR. Size (measured in head to tail length) seems to be the best predictor of the onset of the JAR. A distinguishable JAR value (0.15 Hz or greater) develops in fish at a length of 12-15 mm, and its strength continues to increase with maturity until it approaches an adult value (8-20 Hz) at a length of about 45 mm. The JAR is not dependent upon social interactions, as it can be performed correctly upon first stimulation by animals raised in individual aquaria from the egg stage. Preliminary studies suggest that there are anatomical correlates to the development of the JAR behavior. As the JAR strengthens with age, there is a concomitant increase in the number of giant cells and a development of the commissural plexus in lamina 6 of the torus semicircularis. Giant cells play a pivotal role in the phase comparison circuit. Both phase and amplitude information play a role in the proper performance of the JAR, but the discrete nature of the phase comparison circuit allows the correlation between the development of the JAR and an essential part of the phase comparison circuit (lamina 6 of the torus) to be observed in Eigenmannia.