The effect of aluminium on histopathological and behavioral changes in the planarian Polycelis felina (Daly.) in laboratory conditions was studied. The planarians were treated with seven concentrations of aluminium sulfate for five days and compared to three control groups of animals. Microscopical, histological and morphometric methods were used. The results showed distinguished morphological changes on the planarian body as well as behavioral changes: various depigmentations, disordered locomotion, twisting of the body parts, hardly reacting to the mechanical stimuli, body contractions, deformations and mortality. Histomorphometric analyses showed changes in the size and number of neoblasts and reticular cells. Changes in epithelial cells and segments, parenchyma and muscular layer were also noticed. Concentrations of 600 and 1100 mg/L caused most distinct changes in behavior, morphological and histological structure of planarians. The highest concentrations caused irreversible changes. Considering high LC 50 (1100 mg/L), the used concentrations of aluminium presented no actual environmental threat to planarian populations in nature, but showed the extent of damages in a possible concentrated aluminium environment.