A one-year study was conducted on the life history of the amphipod Corophium insidiosum (Crawford, 1937) in the Mar Piccolo estuary (Southern Italy). Monthly collections were made to investigate certain aspects of population structure, abundance and reproductive biology. Population density showed a clear seasonal variation: with a maximum in spring-summer and a minimum in autumn-winter. Although brooding females were present all year round, recruitment occurred in spring, decreased in summer, peaked in autumn and almost ceased during the winter. 7-8 new cohorts in all samples could be recognised from April 2002 to November 2002. Mean longevity was ~5 to 6 months, and the estimated lifespan was longer for individuals born in late summer than for individuals born in spring. The sex ratio favoured females with a mean value of 1.51, but males grew faster and attained a larger maximum body length than females. Males and females became distinguishable at roughly > 2 mm, reaching a maximum size of 5.6 mm for females and 6.0 mm for males during the winter months. The females reproduced for the first time when they reached 2.2 mm body length. The number of eggs carried by females was related to the size of the female.