Bacterial mat is commonly found on the fl oor of Lake Nakaumi from late spring to late autumn. Particularly bacterial mat develops in a low oxygen condition（dysoxic to anoxic environment）of bottom water and without the bottom water turbulence. In such a bottom water condition, a fi sh stripped off the bacterial mat and consequently a muddy lake fl oor appeared in the size of about 16 cm diameter. However, the bacteria are very productive in the low oxygen water. We observed the recovery of bacterial mat is completed by about 18 hours after that the muddy substrate cropped out.