The density of the cytoplasm of Paramecium was increased by iron-feeding, and the gravitactic and gravikinetic behaviour of the cells was analyzed. For determination of gravikinesis, upward and downward swimming rates, cell orientation and sedimentation rates were evaluated using computer-aided video analysis. Iron-fed cells showed increased sedimentation rates as compared to control cells. The precision of graviorientation was increased in iron-fed cells and their swimming rates were larger than the swimming rates of controls. Moreover, iron-fed cells showed larger negative values of gravikinesis than control samples.