Abstract: Sturgeon is an ancestor electrosensory fish which is able to sense the electric fields of prey animals and other sources. The character of bioelectric fields in environment is an important stimulus to investigate the electrosensory information processing in the brain. However, there were a few documents on bioelectric fields. In this study, the bioelectric fields of Gambusia affinis were measured in living fish, which was used as prey fish of Siberian Sturgeon in laboratory and the common small fish in southern China. They were measured and charactered in three experimental groups: single fish group, pair in same direction group and pair in opposite direction group. The bioelectric field of single fish showed typical features of dipole field, with relative potential up to (242.4) V on head and (211.6) V on tail. There was (4.20.8) V alternating current electric with 1-3 Hz on head which was corresponded with the respiratory rhythm of the fish. In other two experimental groups, the direct current electric fields were larger than the single fish group (P0.05). However, the alternating current electric field of single fish group was significant bigger than pair in opposite direction group (3.80.7) V but smaller than pairs in same direction group (4.50.8) V (P0.01). It suggested that the schooling fish could evade the predators by modulating the alternating current respiratory potential. It was maybe a kind of ancestral antipredator strategy for fish to meet the passive electroreception, and further research would carry on electrophysiology.