Population declines are noticeable when adults disappear locally or massive die-offs are reported. However, populations can slowly decline if recruitment is unsuccessful. We aimed to determine the recruitment rates and the biotic factors involved in the metamorphosing success of a threatened species, Melanophryniscus montevidensis. Using in situ enclosures in ponds of a protected area (Laguna de Rocha, Rocha, Uruguay), we assessed larval survivorship, body size, and duration of development, from eggs to the end of metamorphosis. Also, we evaluated which factors of the larval dynamics (density, body size, predation, location, and depth) better explain the overall survivorship using generalized linear models (GLMs). We recorded metamorphs in only 8 of 25 clutches, accounting for a median success per clutch of 1.2%. Median development time until Gosner stage 42 was 26 days, with a median body length of 6.1 mm. The best simplified GLM for survival success included number of larvae (recorded 10 days before sampling) and depth as significant negative predictors and body length as a positive predictor. Depth and body length had the greatest effect. The body size at metamorphosis was negatively correlated to development time, suggesting a delayed growth possibly involved in the lack of response to pond duration (causing mass mortalities) in some enclosures. In spite of the potential plasticity of the species to respond to pond desiccation, we predicted that ponds that dry in less than 20 days might not yield any offspring. The in situ data provided by this study may help understanding the processes behind the decline of M. montevidensis, thus helping to establish proper conservation actions for the populations and the ponds where they breed.