ABSTRACT. A branchial infection of Pecten maximus is histologically and ultrastructurally described. Gill epithelia1 cells were heavily infected with colonies of procaryotes, believed to belong to the Rickettsiales.In March 1987, a mass mortality (ca 40 O/O) of Pecten maximus from St Brieuc, N Brittany, France, was reported. Collections were made at this site in order to conduct an anatomopathological study, which revealed a procaryotic infection of gill endothelial cells.Materials and Methods. Adult and juvenile scallops were collected by dredging at St Brieuc. Tissues were fixed either in Carson's fixative for light microscopy or in glutaraldehyde 1.25 %/paraformaldehyde 2 % in 0.1 M sodium cacodylate/HCl buffer pH 7.4, followed by 1 % osmium tetraoxide in the same buffer, for transmission electron microscopy (TEM). Osmolarity was raised to 1100 mosm with sucrose (primary fixative) or with sodium chloride (secondary fixative). For light microscopy, samples of gills, digestive gland, mantle, kidney and muscle of a first set of 25 individuals were processed and embedded in paraffin; 3 pm sections were stained with hematoxylin and eosin. A second set of 50 individuals was treated as above except that only gills and digestive gland were examined.For TEM, gill tissues were prepared using an automatic processor and embedded in LX 112. Sections (60 to 90 nm) were contrasted with lead citrate and aqueous uranyl acetate by means of the LKB Ultrostainer. Grids were examined with a Jeol 1200 CX electron microscope.

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