We studied a critically-endangered hydrobiid snail, Pyrgulopsis bernardina, in Snail Spring, Cochise County, Arizona. We determined the relationships between its abundance and a suite of habitat variables and document formerly-occupied sites. Using Akaike's Information Criterion for model selection and averaging, 14 models were defined a priori using eight habitat variables to explain its abundance. All biologically supported predictors, in the first order, provided the most parsimonious explanation of P. bernardina densities. Structural variables received stronger support than physicochemical variables. P. bernardina densities were positively associated with sand and cobble substrates, vegetation density, water velocity, and dissolved oxygen, as well as water temperatures of 14-22°C and pH values between 7.6 and 8.0. Snail densities were negatively associated with silt and organic substrates, deep water, and high conductance. The current and all former localities for P. bernardina are associated with a shallow aquifer that is distinct from an adjacent deep aquifer.