We analysed several life history traits of the marine isopod Idotea balthica basteri (Pallas, 1772) from the Bizerte lagoon, Southern Mediterranean Sea. Growth was continuous throughout the life of the animal with a high growth rate in the first life phase, and the growth curve was described according to von Bertalanffy's model. The lowest growth rate (0.23 mm) was recorded in winter (December, January and February) and the maximum rate (2.31 mm) between April and June. The total number of hatched eggs or embryos was positively correlated with the body length of ovigerous females. This population of I. balthica basteri was iteroparous, showing distinct strategies of reproduction. Large ovigerous females with high fecundity were collected during the whole sampling period, while breeding in smaller females with low fecundity was restricted to the period from late spring to early autumn, Manca size increased significantly with increasing female body size and there was also a significant trade-off between manca size and the number of eggs per brood. Reproductive allocation, ranging between 17.1 ± 1.2% in winter and 23.2 ± 1.8% in summer, was positively correlated with female weight. Accordingly, parental investment in producing a juvenile varied between 1.02% per manca in winter to 3.38% in spring. Evaluated traits show that late summer and autumn cohorts have a K-strategy, whereas cohorts born in winter and spring, and which exhibit a shorter life time, exhibit faster development, earlier reproduction and a smaller parental investment tending towards an r-selected strategy.