This paper describes the life cycle of the amphipod Jassa marmorata with data recorded over two years (from May 2002 to April 2004) in the Marine Protected Area of Portofino (Ligurian Sea, Italy). The population was sampled monthly at 5 m depth on artificial substrata. For each sample, the number of males, females, ovigerous females, and juveniles was recorded and related to water temperature. Two kinds of males, minor and major, were found, distinguishable by the presence of a thumb on the second gnathopod. The sex-ratio was significantly biased towards females (F:M = 5.6:1). The cycle revealed a female increase in mean body size during the cold season (late winter and early spring). This resulted in a peak in recruitment during the spring; females are fertile all through the year, but egg production is positively correlated with female size. During the summer of 2003, an anomalous increase in sea-water temperature was recorded. In this period the population completely disappeared, but the density during the next winter recovered again to usual values.