Summary To investigate in real time if and how natural gravity affects rates of swim‐up and swim‐down of human spermatozoa, samples of motile or immobilized spermatozoa in a sealed mini‐chamber were placed vertically on a 90° tilted microscope. The mode of their sedimentation, as well as the difference in the rate of their swimming up and down, were observed directly over 30 min and analysed from photomicrographs. Under the influence of natural gravity force, most immobilized spermatozoa turned their heads down in about 5 min and then sank slowly at an average speed of 0.2 μ/s. The number of motile spermatozoa that swam down was 5–6 times more than those swimming up. It can be implied that in spite of the mild force exerted by 1g on suspended spermatozoa in comparison to the high g force obtained by centrifugation, the overall effect of gravity on the rate of swimming up or down becomes dominant. Gravity causes the sperm heads to turn downward after which the oriented spermatozoa continue to move down by their own tail movements, causing accumulation of motile spermatozoa at the bottom. This may explain why in some recent studies swim‐down was superior to the swim‐up procedure during sperm separation by self‐migration.