We quantified abundance and biomass of the amphipod Gammarus chevreuxi Sexton from benthic cores, sampled monthly over 1 y in the upper reaches of Canal de Mira, the southern arm of the Ria de Aveiro, Portugal. Abundance and biomass showed no clear seasonal patterns but were associated with variation in salinity, dissolved oxygen, and chlorophyll a concentration. Maximum abundance and biomass occurred at relatively low temperature and high food availability, conditions that likely increased survival and/or reproduction. The population showed a semiannual, iteroparous life cycle. Mean life span was ∼6 mo, with overwintering individuals hatched in autumn showing higher longevity than individuals hatched in spring. Breeding was continuous, although juvenile recruitment peaked in early autumn, winter, and early spring. Sudden fluctuations in abundance and size structure of the population may have resulted from immigration during autumn and emigration during winter. Mean fecundity (8 embryos/brood) and intramarsupial loss (0–18%) were low compared with studies of other Gammarus species. Variation in fecundity and intramarsupial loss was mostly explained by size of incubating females. The volume of the embryos was highly variable but not significantly correlated with female size or other reproductive traits. The Hynes and the Morin– Bourassa methods yielded similar estimates of annual production (∼46 g AFDM m⁻² y⁻¹) and P/B̄ ratio (11/y).