Considering the filtration capacity of mussels to qualitative particle selection, our study focused directly on ingested species by assessing mussel stomach content of various size classes (1-5 cm) over a year (August 97-June 98) rather than monitoring seawater samples. Different size of mussels (1, 2, 3, 4, 5 cm in length) was sampled monthly from the sides of fish cage floats in Urla-Izmir. Stomach contents were analyzed under microscope and different plankton species were grouped taxonomically. Bacillariophycea, Dinophyceae, Oxyphceae, Euglenophyceae as phytoplanktonic species were observed in the stomach contents of mussels as well as zooplanktonic species including Ciliata, Cladocera, Copepod, bivalve and gastropod larvae. Over the sampling period, according to the Bray-Curtis similarity test mussels' stomach for all sizes was of similar content except in August and October.