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We determined from fish collections made along the Atlantic and Gulf coasts that adult menhaden, genus Brevoortia, which school with juvenile menhaden in estuarine nursery areas are usually injured or are parasitized with the isopod Olencira praegustator, whereas adults in the ocean, bays, and sounds are undamaged. Olencira praegustator appears to be debilitory to menhaden, but the copepod Lernaeenicus radiatus does not, except when the fish are heavily infested. When many injured and parasitized adult menhaden are present in an estuary, they form schools independent of the juveniles. Large injured juveniles remain in the nursery areas longer than similar-sized healthy juveniles, and school with the remaining small juveniles.