Giant nerve fiber conduction velocities were studied using noninvasive electrophysiological recording techniques in adults from 12 species of oligochaetes, representing five different families. Two separate and stereotyped all-or-none response patterns to tactile stimulation (corresponding to medial and lateral giant fiber spike responses) were observed in all 12 species, suggesting that this functional dichotomy has been a conserved feature in the evolution of oligochaete escape reflexes. The results indicate that there is considerable species specificity with respect to medial and lateral giant fiber conduction velocities, which may derive from species differences in genetically determined limits in the radial growth of the giant fibers.