The size-specific rate of shell dissolution was measured in situ in three Michigan Lakes that differed in pH and calcium concentration using shells of two species of freshwater pulmonate snails, Physella integra (Haldeman 1841) and Helisoma anceps (Menke 1830). Lack of a periostracum resulted in significantly higher rates of dissolution as did exposure to lake water with either low pH and/or low [Ca+2]. Three factor ANOVA indicated significant effects for periostracum condition and site as well as significant two and three-way interactions. For H. anceps (but not P. integra) the periostracum appears to greatly reduce the rate of shell dissolution hence may serve to reduce shell dmaage in acid waters. Adverse water chemistry (low pH and/or low Ca+2) has a much greater effect on shell dissolution across species (CD = 81%) than does presence or absence of periostracum (CD = 4%). Shell dissolution response to water chemistry as well as the degree of periostracum protection differs between these two species of snails. Models of lake acidification involving either Ca+2 flux and pool sizes or mollusk population survival may need to consider such differences.