The histopathology of Pseudoterranova decipiens (L3) in experimentally infected rainbow trout (Oncorhynchus mykiss) and naturally infected Atlantic cod (Gadus morhua) was similar. The chronic granulomatous inflammatory reaction included polymorphonuclear neutrophils, macrophages, lymphocytes, epithelioid cells, and fibroblasts. Giant cells were also found but only in the experimentally infected rainbow trout. Mature capsules around the larvae consisted of an inner layer, composed of macrophages which underwent epithelioid transformation and later gradually degenerated, and an outer layer, composed of fibroblasts and collagen fibres. A layer of lipofuscin was adjacent to the parasite in older cod infections but this was absent in all of the newly formed capsules in the experimentally infected rainbow trout.