Interspecific feeding interference between neighboring suspension-feeders is an important but understudied structuring process in marine benthic communities. The comparison of δ13C and δ15N stable isotope ratios (SIR) between species is widely used to identify the dietary overlap and competition for food. The results this approach provides are sometimes substantially biased by the variation of isotope fractionation rates between species and tissues. We suggest that the difference in SIR within a single species in the presence and absence of a potential competitor provides a stronger evidence of feeding interference. In the White Sea shallow subtidal solitary ascidians Styela rustica (L.) frequently develop clumps on horse mussels Modiolus modiolus (L.). Within 0.25 m2 of the bottom the horse mussels without ascidians and ascidian clumps attached to gravel are also common. We analyzed SIR in muscular tissues of neighboring Modiolus with and without ascidians attached and in Styela from immediately close clumps