We studied the feeding ecology of juvenile lumpsuckers, Cyclopterus lumpus, during their first year in natural floating seaweed clumps and experimental floating seaweed branches, in southwest Iceland. The juveniles appeared to ignore sessile, slow-moving, and small animals (e.g., ostracods, bivalves, gastropods, oligochaetes, polychaetes, turbellarians, rotifers, and nematodes), while taking most other prey organisms in approximate proportion to availability. Juvenile lumpsuckers in floating seaweed fed mainly on prey organisms found on the seaweed but also consumed organisms from the plankton. Ontogenetic changes in the diet were related to size of prey. Lumpsuckers began to feed while they still had some yolk, primarily on small prey, including crustacean larvae and halacarid mites. After they completely absorbed their yolk, juveniles first fed largely on harpacticoids, but amphipods, isopods, and smaller juvenile lumpsuckers formed an increasing proportion of their diet as they grew. Behavioral selection of prey, rather than an increasing ability to ingest larger items, is suggested to be the basis for ontogenetic changes in diet.