We examined the feeding activity of planktotrophic larvae of the eastern oyster Crassostrea virginica on different size particles suspended in surface waters of 2 subestuaries of Chesapeake Bay, USA. Particle suspensions were characterized uslng particle counters and microscopic observations. At both sites, particle assemblages were dominated (in terms of particle number and volume) by particles (predominantly plankton cells) with equivalent spherical diameters <5 pm. Feeding experiments demonstrated that small (<l50 pm shell length) and large (>200 pm) oyster larvae typically ingested particles between 0.5 and 12 pm. However. in the presence of blooms of large (10 to 30 pm) dinoflagellates, small and large larvae extended their maximum ingested particle size to about 16 and 30 pm, respectively. These results indicate that oyster larvae generally exploit foods within the characteristic particle biomass peak present in these estuaries and opportunistically feed upon periodic blooms of large plankton cells. Larvae derived a large percentage (range 20 to 90%) of the total ingested particle volume from picoplankton-size (here defined as 0.5 to 3 pm) particles. These small particles constituted a larger percentage of the material ingested by small larvae than that ingested by large larvae. By comparison, both sizes of larvae derived about 2 to 40% and about 5 to 20% of total ingested food volume from 3 to 5 pm and 5 to 10 pm particles, respectively. In experiments with abundant dinoflagellates >l0 pm in size, both sizes of larvae derived 20 to 30% of their total ingested volume from these particles.