diate hosts (Hopkins, 1954, Journal of Parasitology 40: 29-31; Wardle, 1980, Bulletin of Marine Science 30: 737-743). Adult worms develop in hindguts of marine fish, Archosargus probatocephalus (Walbaum), which feed on hooked mussels (Wardle, 1980, loc. cit.). Although metacercariae are well developed in I. recurvum, progenesis has not been reported for P. maculatus from warmer waters of the southern United States. The present study describes the interactions between hooked mussels and metacercariae. During May 1984 hooked mussels were collected from an oyster reef in Calcasieu Lake, Louisiana (average salinity 17.2 ppt), taken to the laboratory, and their soft-parts examined with a stereomicroscope. Trematode specimens were removed and studied live. Small pieces of host organs containing metacercariae were fixed in Bouin's solution, prepared by routine histological procedures, serially sectioned at 7 gtm and stained with hematoxylin and eosin. Yellow pustules marked sites of metacercariae in the mantle, gonad and digestive gland of the host. When larvae were removed and examined, their intestinal ceca contained host cells that were regurgitated under coverslip pressure. Histological sections showed larvae encapsulated within granulomas containing infiltrates of host leukocytes (Fig. 1). Although many were ingested by larvae, leukocytes were responsible for the "yellow pustules" and were so numerous as to form large clumps after fixation (Fig. 2). Capsule walls were thin and contained a loose network of extracellular fibers (Fig. 2). Host-parasite interactions were similar to those reported by Tripp and Turner (1978. In Comparative pathobiology, Vol. IV, L. A. Bulla and T. C. Cheng (eds.). Plenum Press, New York, pp. 73-84) for M. edulis infected with adult P. maculatus. As in the present study those authors described a leukocytic response in which host cells migrated and surrounded worms only to be ingested by them. However, the response of M. edulis apparently did not involve encapsulations of parasites with concomitant infiltrations of leukocytes through thin capsule walls. This research was supported by a grant from the U.S. Department of Commerce, Office of Coastal Zone Management.