Food preference of the molluscivorous polychaete Halla okudai were examined in field sampling and laboratory experiment. Bivalves such as Musculs senhousia, Mytilus edulis, Ruditapes philippinarum, Crassostrea gigas and Notochione jedoensis dominantly inhabited the intertidal flat, Hiroshima Bay, in which the polychaete was abundant. The preference for these five species of bivalves were indicated by IVLEV's electivity index (E), based on the number of bivalve eaten during 30 days. Mean values of the electivity index were as follows; 0.61 in R. philippinarum, -0.49 in M. edulis, -0.85 in M. senhousia, -0.90 in N. jedoensis and -1.00 in C. gigas. R. philippinarum was more significantly selected than the other four bivalves. As for the preference for different size of R. philippinarum eaten during 60 days, electivity index was calculated on small-, middle and large-sized bivalves, respectively; 0.05, -0.05 and -0.29 in small polychaete, -0.10, 0.02 and -0.02 in middle polychaete, -0.14, -0.11 and 0.13 in large polychaete. Thus, the polychaete tended to feed on large bivalve with increased age.