Food limitation in larval growth of the spionid polychaete Polydora ciliata was examined in a typical eutrophic estuary, Isefjord, in Denmark. In the field, food availability and the energetic requirements of the P. ciliata larval population were measured during 2 different periods in 2004 and 2007 that together cover the productive part of the year for plankton. In the laboratory, specific growth rates (mu) of larvae reared on natural food suspensions (similar to 0.10 d(-1)) were always lower than those of larvae reared on phytoplankton-enriched food suspensions (100% retention efficiency for Rhodomonas salina; similar to 0.21 d(-1)). Total meroplankton biomass (average: 3.72 mu g C l(-1), range: 0.11 to 26.05 mu g C l(-1)) was frequently similar to or exceeded that of holoplankton (average: 5.70 mu g C l(-1), range: 0.08 to 29.89 mu g C l(-1)), suggesting a trophic significance of meroplankton in the estuary. P. ciliata was commonly the dominant meroplanktonic larvae (average: 0.5 mu g C l(-1), range: 0.03 to 2.51 mu g C l(-1)). The available food in the optimal prey size fraction (2004, average: <20 mu m; range: 99 to 274 mu g C l(-1); 2007, average: 7 to 18 mu m; estimated carbon demand: 119 mu g C l(-1); range: 19 to 474 mu g C l(-1)) seemed to be sufficient to cover the energetic carbon requirements of the population throughout the study (0.09 to 3.15 mu g C l(-1) d(-1)), but insufficient to attain the maximum specific growth rates reported in previous laboratory experiments. This suggests that P. ciliata larvae probably exhibit a low feeding efficiency and their maximum specific growth rates are consequently attained at food concentrations even higher than those found in this eutrophic environment.