Pyrgulopsis trivialis is a western North American hydrobiid snail that is a candidate for federal listing under the Endangered Species Act as threatened or endangered. No literature regarding population size estimates for this species were found. This type of information is limited for aquatic gastropods, springsnails in particular, though it is important for addressing conservation issues. Density counts were used on samplers made of saltillo tile within a single springbrook to estimate population size of P. trivialis. Three population estimates were obtained over the study period in 2002: 129,135; 80,130; and 164,234 snails. These estimates were statistically comparable, though they may under-estimate the true population size. Nevertheless, this provides a reliable and repeatable method of estimating population size for P. trivialis and other hydrobiids that live in similar lotic habitats.