ABSTRACT. – Porcellio lamellatus, a supralittoral terrestrial isopod, is frequent along Atlantic river side’s and Mediterranean coasts. Biology, population dynamics and life cycle were studied by monthly surveys performed from June 2006 to May 2008 on the supralittoral zone of Bizerte lagoon. Ovigerous females were present from February/March to October/November, suggesting that Porcellio lamellatus has a seasonal reproduction pattern. Female size was positively correlated with fecundity (number of eggs per brood) and fertility (numbers of embryos hatched per brood). Sex ratio was globally in favor of females (0.86). Furthermore, analysis of the size frequency polymodal distribution showed that five cohorts were identified in the first sampling date and nine new cohorts were detected during the next months. The average length of newly born cohorts ranged from 1.39 ± 0.26 mm to 2.93 ± 0.7 mm. The estimated lifespan ranged from 4 to 8 months. Porcellio lamellatus behaved as a semi-annual species, with iteroparous females, and a bivoltine life cycle.