In the present studies, the behavior of ddY mice digging wood chips was carefully observed. When mice were individually placed on new 5 cm-thick wood chips, their behavior was found to be the same irrespective of their age or sex. The behavior was not prevented by non-noxious 5 black steel rods which were used to measure digging ability, and was not related to habituation or learning. But moist or dirty chips remarkably weakened digging ability. These findings strongly suggest that the digging behavior is a natural and instinctive one, but not an expression of anxiety as previously reported.