Isopods associated with prop roots of Rhizophora mangle were collected in a coastal lagoon system of western Mexico. The system, Estero de Urias, has no permanent freshwater supply and is under strong influence of tides. Four species were recognized. Paracerceis sculpta and Cassidinidea mexicana were rare but Paradella dianae and Uromunna sp. were occasionally abundant and widely distributed within the system. There was a clear tendency for P. dianae to occupy roots in the outer (closer to the inlet) portion of the system, while the opposite occurred with Uromunna sp. Both species were tolerant to some variations in salinity (31-38 ppt in P. dianae; 31 to 42 ppt in Uromunna) and temperature (24-33°C in P. dianae; 21-32°C in Uromunna), but a drastic drop in salinity during the rainy months had a negative effect on absolute abundance of both species. Forty-two combinations of major sessile epibionts were recorded on prop roots. Uromunna sp. was found associated with 18 of these combinations and P. dianae with 12, although the presence of these two species in some communities was marginal. A one-way ANOVA indicated that macroalgae, alone or combined with other epibionts, were statistically more important for Uromunna sp., while barnacles (alone or combined) were more important for P. dianae. The sex ratio in Uromunna sp. was relatively stable during the entire survey (32-49% were males), except in June. A large proportion of gravid females (9-38%) was observed from October to July and the species appears to reproduce year round. Males of P. dianae were always dominated by females and/ or juveniles in number, and usually represented less that 30% of the specimens. Juveniles were present almost all year round. The largest size for Uromunna sp. was observed in March and July; the smallest size in November, December, and April. For P. dianae, the largest size was observed in April and October, the smallest in March, August, and December.