The size of a bacterial cell is so small that its swimming motion is gov-erned by viscous forces rather than inertial forces. The Reynolds number, the ratio of the inertial forces to the viscous forces, is estimated to be 10"^. If the motor of a singly flagellated bacterial cell changes its rotational direction from CCW to CW, the cell instantaneously switches its swimming direction. Fur-thermore, the forward and backward swimming is expected to be symmetrical. In a highly viscous fluid, the flow is reversible; the direction of flow cannot be distinguished by only observing the stream lines. If an object moves in one di-rection and is then moved back along the same path in the opposite direction to the previous starting position, the total momentum imparted to the neighboring fluid will be zero and the object will not gain any net momentum. The actively bending flagella of eukaryotic cells change their shapes between effective strokes and recovery strokes to generate a net momentum. By con-trast, the filament of a singly flagellated bacterial cell does not change its shape between forward and backward motions. Therefore, the cell is expected to follow the same path as long as there are no disturbances like Brownian motion. Despite this assumption, previous experimental works have revealed that there are asymmetrical characteristics between forward and backward motions of singly flagellated bacterial cells (Magariyama et al. 2001, Kudo et al. 2005, Magariyama et al. 2005). These differing characteristics have been observed in swimming speed and trajectory. However, the asymmetrical motion ap-pears only when the bacterial cell swims close to a surface. In the following sections of this paper, the experimental results will be described, and an ex-planation based on fluid dynamics for the cause of this peculiar motion near a surface will be detailed.