Laboratory experiments were carried out to evaluate juvenile and adult mortality in Musculium argentinum over a given period of time. Specimens of M. argentinum were collected in May 2009 from the evacuation channel of the Lautaro fish farm, Araucanía Region, Chile (38°32′ S, 72°27′ W), and a laboratory experiment was carried out consisting of 2 treatments (T1 and T2), with 3 replications of each. In each treatment and replication, a plastic container (15×10×5 cm) was filled with water and dead leaf litter from the channel inhabited by the study species. During T1, the population density of the container was 100 individuals (D100); during T2, the population density was 60 individuals (D60). The individuals were allowed 2 days to adapt to their new environment. Every 7 days during the following 2 mo, the anteroposterior valve length of each specimen in each treatment was measured to an accuracy of 0.1 mm. Adults were measured using digital calipers, and juveniles were measured using a stereomicroscope fitted with a micrometer eyepiece. Mortality was determined by counting and measuring the number of empty left valves of adults and juveniles. The mortality rate (MR, measured as a percentage) was estimated by counting the number and recording the size of live individuals (LI) present at the start of each period, the number of recruits (R) entering the system during the period, and the number of dead individuals (DI) during the same period using the formula MR = (DI/[LI R]) × 100. From the results of the current study, it may be concluded that M. argentinum populations have a mortality rate that varies over time, with juveniles showing a higher mortality rate than adults, and that the population density affects significantly the mortality of both classes of individuals. The results also show that mortality in M. argentinum is density dependent.