ABSTRACT. – The population structure and dynamics of Porcellio laevis Latreille, 1804, a terrestrial isopod, were studied in its natural population at Wadi Joumin in Bizerte (Northern Tunisia), over 18 months from February 2013 to July 2014, by monthly sampling. The ovigerous females were only present from March to August. This species exhibits seasonal reproduction followed by sexual rest. Fecundity and fertility were both estimated to better understand the different steps of reproduction from sexual rest to manca release. Results showed a highly significant difference between oocyte incount during sexual rest and the first step of reproduction. Then, a non-significant difference between both steps of (eggs-embryos) and (embryos-mancas) was obtained. The sex-ratio was female biased and a high significant difference was statistically observed. Analysis of the size frequency polymodal distribution suggested the presence of four cohorts on the first sampling date and 7 new ones during the next 17 months. Lifespan was estimated at 10-12 months. At this study site, P. laevis appeared as a semiannual species, with iteroparous females producing at least two broods, and exhibited a bivoltine life cycle.