A new method for labeling planorbidae to estimate the density and other parameters related with population studies, such as migration capacity, growth rates and biotic potential in natural habitats, is proposed. The snails are labelled with two small holes (2 mm diameter each) perforated in each side of the shell by means of a pointed metalic tool. After labeling, the snails are immediately put back in their habitat, at the same site where they were captured a few minutes ago. In all experiments carried out, the rates of labelled snails, recovered a week after labeling, varied from 14 to 18% and allowed to estimate the total population of B. glabrata, with 5 mm diameter or more. This method enables to evaluate juvenile population (with less than 5 mm), as well as to perform different marks. It is very simple/ease of handling; it does not require either specialized personnel or equipment and, on the other hand, it causes less trouble in the habitat than other procedures do.