ences the motion of the cell. If the cell body swims backward toward the sur-face, the interaction between the flagellar filament and the surface is stronger than that between the cell body and the surface. Thus, both the swimming speed and the trajectory depend upon the attitude of the cell.

 Numerical simulation

 Boundary element analyses (Ramia et al. 1993) were carried out to investigate the effect of the fluid-dynamic interaction between a bacterial cell and a rigid surface on the asymmetrical motion illustrated in previous sections by chang-ing the cell's distance from a rigid boundary and the attitude of the cell (Goto et al. 2005). Since the Reynolds number of the flow associated with the mo-tion of a bacterial cell is very small, it was assumed that the fluid motion is steady at any instant and is a creeping flow governed by the steady Stokes equation. The velocities and the angular velocities of the cell body were cal-culated from the condition that the net force on the whole cell is zero and that the net torque is also zero. In order to take into account a rigid surface that produces a no-slip boundary condition, a basic solution derived by Blake (1971) was adopted. The dimension of the cell model was determined according to the meas-ured values of YM4 cells (Goto et al. 2001). The calculations were performed by changing the pitch angle 9 and the distance d shown in Fig. 8. The veloci-ties and the angular velocities were obtained for each set of 0 and d. The swimming speed was defined as the velocity in the x direction Ux. The pitch rate Qy and the yaw rate Qz" were also examined. Here, the yaw rate was de-fined as the projection of Qz' to the z direction, as shown in Fig. 8. The swimming speed Ux, the pitch rate Qy and yaw rate flz" of the cell model swimming forward are shown as functions of the pitch angle 0 in Fig. 9(a)-(c), respectively. These values are normalized by the absolute values of the X component of the swimming velocity vector |Uoo| and the x component of the angular velocity vector \Qco\ of the model swimming in free space. Note that, in the coordinate system defined in Fig. 8, the forward swimming direction is in the negative x direction. Figure 9(a) shows that the swimming speed is unaffected if the cell swims at a distance of around ten times the diameter of the cell body (see d/2b=12.5)