Forty-three wild rats (Rattus norvegicus) trapped in Milan (Italy) were examined for gastric spiral bacteria and to evaluate associated histological lesions. Spiral bacteria were histologically detected in the stomach of 10 rats (23%). The morphological features of these microorganisms, observed using both light and electron microscopy, correspond to those of Helicobactem heilmannii. Mild gastritis characterized by focal lymphoplasmacytic infiltration of the lamina propria was observed in six rats with gastric spiral bacteria. Our findings suggest a causal role of these bacteria in inducing a gastric inflammatory response.