Multimammate rats of the genus Mastomys are among the most widespread pest species in Africa. Previous studies of Mastomys population dynamics have generally reported variation in abundance but few have investigated the demographic parameters underlying this variation, and in particular recruitment. Capture-mark-recapture data were collected for Mastomys erythroleucus several times a year from 2000 to 2004 at a site annually flooded by the Niger River in Mali. Closed-population models were used to estimate population abundance. Both seniority (a parameter inversely linked to recruitment) and survival probabilities were estimated by capture-mark-recapture models. The impacts of water level, population abundance and cumulative rainfall were assessed for each demographic parameter. Survival probabilities (local survival) were negatively correlated with water level, suggesting that rodents emigrated out of the study zone during flooding. As for seniority probabilities, 86% of temporal variation was explained by a model with season, abundance, water level and the interaction between abundance and water level. This suggests that density-dependence in recruitment was mediated by intraspecific competition for food or refuge from floodwaters, or by predation. The flood of the Niger River greatly impacts Mastomys erythroleucus population dynamics, affecting both survival and seniority probabilities.