Vertical dispersion of the cercariae Renicola thaidus in the water layer was examined in experiments. Dispersion of larvae of 1 and 6 hours old were investigated in different conditions of illumination. It was found out that in the first hour of life the larvae both in light and dark conditions display a negative geotaxis. 6 hours later, the reaction alternates. Both in light and dark condition, the larvae show a positive geotaxis. This changes of vertical dispersion is the result of inversion of the reaction onto the gravitation, that depends on biology of cercariae examined.