The adverse effects of heavy metals (mostly Cr, Mn, Fe, Cu, Zn, and Se) on the environment were appraised by comparing fluctuating asymmetry of dental characters (molars) in Algerian mice (Mus spretus) from polluted (Oeiras) and reference (Xévora) riparian areas, in central Portugal. The size of tooth traits was shown to be reduced in the contaminated area, and the developmental instability increased. The results suggest that Algerian mice may be used as indicators of heavy metal pollution and that fluctuating asymmetry represents a useful tool for monitoring ecological stresses, even when the contamination levels are low.