Aspects of the biology of Gammarus aequicauda were studied at extremely low salinities (0.3-5.7 psu) in Monolimni Lagoon, N. Aegean Sea. Samples were collected monthly from February 1998 to February 1999. Breeding occurred continuously but peaked in late spring, late summer and autumn and three cohorts were produced. The spring and summer cohorts showed fast growth (0.15 mm d -1 ), accelerated maturity and life span of about three and seven months respectively, while the overwintering cohort showed a life span of about nine months. The largest individual had a body length of 23.6 mm. Mean brood size was 54.5 early embryos, while the intramarsupial loss was 46%. Population density sharply increased in late spring, summer and autumn. Secondary production calculated by Hynes' method gave a mean annual density of 1077.4 ind. m -2 , a mean annual crop (B) of 2.93 g DW m -2 , an annual production (P) of 35.03 g DW m -2 and a P : B ratio of 11.96. Gammarus aequicauda showed a life-history pattern similar to those previously reported for this species at higher salinity environments with comparatively large final body length and high growth rate during summer, brood size and P : B ratio, but with high embryo loss as well.