Stomachs of plaice and sole were collected in 1996 within and just outside the 'plaice box' (PB), an area in which fishing by vessels larger than 300 hp has been prohibited since 1989. In the mid-1990s the beam trawl fishing effort was reduced by 85␘f the pre-closure level. In addition, a comparison was made of the diet composition of plaice and sole between the present and the beginning the 20th century. The diet of both species comprises mainly short-lived, highly productive benthic organisms. No difference could be found between the diets of fish sampled at grounds with different trawling intensities. The comparison of the present-day diet and the diet at the beginning of the 20th century suggests that the preponderance of polychaetes has increased and that of bivalves decreased. These results are consistent with the hypothesis that beam trawling has improved the feeding conditions for the two flatfish species by enhancing the abundance of small opportunistic benthic species such as polychaetes in the heavily trawled areas. However, the changes in diet may also be related to eutrophication and pollution.