  The collective motion of bacterial cells in a shallow circular pool is
systematically studied using the bacterial species $Bacillus$ $subtilis$. The
ratio of cell length to pool diameter (i.e., the reduced cell length) ranges
from 0.06 to 0.43 in our experiments. Bacterial cells in a circular pool show
various types of collective motion depending on the cell density in the pool
and the reduced cell length. The motion is classified into six types, which we
call random motion, turbulent motion, one-way rotational motion, two-way
rotational motion, random oscillatory motion, and ordered oscillatory motion.
Two critical values of reduced cell lengths are evaluated, at which drastic
changes in collective motion are induced. A phase diagram is proposed in which
the six phases are arranged.
