Distribution, population dynamics, growth and aspects of reproductive biology of Corophium insidiosum were investigated in Monolimni lagoon. Samples were collected in July 1997 (at 30 psu S) and during February 1998-May 1999 (at 0.1-5.7 psu S). Corophium insidiosum was almost exclusively found in the outer part of the lagoon, which showed a higher water renewal rate. Population density gradually decreased during winter and spring, when salinity was lower than 1 psu and the amphipod finally vanished from the lagoon. Salinity increase during summer (1.2-5.7 psu) was followed by the re-occurrence of C. insidiosum with a time lag of 2-3 months. Population density increased in autumn and peaked in early winter at salinities 1.6-4.2 psu. Three cohorts appeared in the population during September 1998-March 1999. Breeding activity peaked in early autumn (14-21 °C, 4 psu S) and ceased after December (2-6.5 °C, <1.5 psu S). The preponderance of females in the large size classes resulted in a female- biased sex ratio in the whole population. The population showed a growth rate of 7.5-11.2 μm d-1 being faster in autumn (9-21 °C, 3-4 psu S) than in winter (2-12 °C, 0.2-3 psu). An exponential relation existed between body length and cephalic length or dry body weight, while brood size was directly related to body length. Mean brood size was small (4.96 early embryos) and egg loss during development high (53%), possibly as a consequence of low salinities.