The population structure and reproductive condition of the neustonic isopod Idotea metallica was determined from samples collected in June 1993 during a plankton cruise off the Catalan coast (western Mediterranean). Sexes were distinguishable from a size of around 4 mm total length. Both male and female size population structure were found to be polymodal. Ovigerous females were found at sizes larger than 13 mm and a mean size at sexual maturity of 14.9 mm was estimated. Brood size varied between 30 and 182 embryos per female and was positively correlated with female size.