We study rheotaxis of bull sperm inside microchannels to characterize the effects of flow and wall shape on sperm swimming behavior. We found that large percentage, 80 to 84%, of sperm cells exhibited positive rheotaxis (sperm cells swimming against the flow) within flow velocities of 33 to 134 µm/s. Sperm cells were also found to reverse their swimming direction when liquid flow direction was reversed. Time taken by sperm cells to reverse their swimming direction was inversely proportional to flow velocity. Sperm behavior was significantly affected by sperm position with respect to channel wall. Sperm cells close to channel wall moved upstream faster than sperm cells moving along channel centerline. Shear stress, which is an indicator of velocity distribution, was found to play an important role in regulating rheotactic behavior of sperm cells. Side pockets were added to some microchannels to mimic storage sites in mucosal folds and pockets in the fallopian tube of female reproductive system and sperm interaction with these pockets was monitored. We found that sperm cells tend to follow channel walls and enter these pockets without any chemical binding, which further confirms the wall tracking behavior of mammalian sperm cells. Our results confirm that sperm rheotaxis is a strong mechanism for guiding sperm cells to the oocyte along the female genital tracts.