Rice seed bank flora in Portuguese rice fields was studied with the aim to understand the population dynamic of weeds. In the present work results of one year of observations in 40 fields are presented. A total of 23 taxa representing 11 families were identified. Among the most important weed species, Heteranthera limosa, H. reniformis, red rice (Oryza sativa), Echinochloa crus-galli and E. oryzicola were detected with relative frequency of 35, 13, 8, 55 and 15%, respectively. Leptochloa fusca ssp. fascicularis, a rice weed recently introduced in Portuguese rice fields, was detected only in Sorraia Valley.