Zebra mussel (Dreissena polymorpha) strongly affects aquatic ecosystems and underwater hydrotechnical equipment. In this study, movement direction of juvenile mussels (< 10 mm) was studied on a glass plate inclined against an aquarium wall (at angle of 16.5° to the bottom), or on the flat bottom, in various light conditions. In the darkness, the mussels exhibited a negative geotaxis, while in uniform light the numbers of specimens moving up and down the plate were similar. On the flat bottom, the mussels displayed a negative phototaxis. When only the upper part of the inclined plate was illuminated, most of the individuals moved downwards, indicating that light was a stronger signal than gravity. On the inclined plate with its lower part illuminated, more mussels moved upwards (like in the darkness), but this tendency was not statistically significant. Due to the observed behaviour, mussels could counteract adverse effects of conspecific competition by selecting sites on the top of a colony (due to the negative geotaxis in the darkness), and avoid dangers associated with water surface, such as increased probability of air exposure or predator attacks (due to the negative phototaxis in the light).