The enchytraeid oligochaeteEnchytraeus japonensisreproduces asexually by fragmentation under laboratory conditions. In the fragmentation process, fully-grown worms break into several fragments. Each fragment regenerates into a small but complete worm in 4 days, grows rapidly and divides again in another 10 days. We found that this fragmentation can be induced artificially by amputating the head if the worms are at least 5–6 mm in length, but not in shorter worms. The fragmentation is inducible by removing the most anterior two segments, if the worms are large enough. When a worm is cut into two, fragmentation occurs more readily in the posterior section. Moreover, even a small incision made in the ventral side of the trunk causes fragmentation in the body posterior to the incision. Immersion of the worms in water is found to inhibit fragmentation even in decapitated worms. When a worm is placed in water immediately after decapitation, the ability to fragment is gradually lost as a new head regenerates. From these results it is postulated that the ability to fragment acquired early in the growth phase is suppressed by head-derived signal(s) until spontaneous fragmentation occurs. The signals seem to be constantly transmitted through the ventral nerve cord until the head matures and lifts its blockade of fragmentation, allowing this process to proceed. At present, however, we cannot exclude the possibility that the mature head of the worm produces fragmentation-stimulating signals.