Four species of isopod characteristic of salt marsh habitats, Tylos ponticus, Porcellio lamellatus, Halophiloscia couchii and Armadillidium album coexist in the upper reaches of the Ria Formosa lagoon salt marsh system in southern Portugal. In this locality, T. ponticus is the most abundant of the four species with mean annual densities of 2,950 m-2 and a peak density of 10,387 m-2 in July 1998 which is very much higher than what has previously been recorded for any isopod in any habitat. The mean annual densities for the other species were lower: P. lamellatus 36 m-2, A. album 19 m-2 and H. couchii 3 m-2, indicating a less significant role in this ecosystem. Tylos ponticus and A. album started to breed on May, 24 and 12 months after release from the marsupium, respectively, where as other species start to breed in March, 12 months after their release from the marsupium. Tylos ponticus has a relative growth rate (RGR) of 0.23 between release from the marsupium and time of first breeding in July of its second year and breeds at a mature mass of 3.6 mg AFDM whereas the other three species mature after 10-12 months, have more than double this RGR but because of the shorter pre-reproductive period breed at masses of 1.8 mg AFDM for P. lamellatus, 1.0 mg AFDM for H. couchii, and 1.1 mg AFDM for A. album, respectively. The mass specific fecundity of all three of the less abundant species was higher than that of T. ponticus but the offspring of T. ponticus were ten times heavier than those of the next largest species, P. lammellatus. The difference in abundances between the species is interpreted as being due to the larger mass of the offspring of the most successful species. This larger mass confers an adaptive advantage due to larger size being associated with reduced juvenile mortality for isopods under abiotically stressful conditions.