In northern Mazovia Helix pomatia L. is rare and found only in some anthropogenic habitats; in the 55 sites studied its frequency was 9%. Neither abundance nor biomass and size structure of its populations departed from those reported from other regions of Poland. Its rarity, the scattered localities and its absence even in apparently optimum natural habitats confirm the earlier view that it is an introduced species in the region. Comparison with the earlier data indicates no expansion tendency within the last 35–40 years. The estimated area occupied by the species in the whole Mazovan voivodeship does not exceed 50 km². These single populations, even when dense, may be threatened because of their isolated character; they should not be exploitedand deserve protection.