An immature didymozoid larva, designated Immature Didymozoid H, is described from the dorsal muscles and swim bladder of Anthias squamipinnis (Peters), a coral-reef fish caught in the Gulf of Aqaba near Eilat. This larva is considerably larger than any didymozoid larva previously described. Protrusion of the larva from the muscles along the dorsal fin of the host fish, and its vigorous, serpentine movements, suggest that "cleaner" fishes may possibly serve as definitive host for the parasite. Among fishes caught 10 km south of Eilat, in the Gulf of Aqaba (Red Sea), six specimens of Anthias squamipinnis (Peters) (Anthiidae) harbored immature didymozoid larvae. These larvae were located in the dorsal muscles of the fishes and in some also on the swim bladder and in the muscles immediately above it. Larvae in the dorsal muscles were found singly, unencysted, protruding, and actively bending half or more of their body in serpen- tine fashion from the dermis near the dorsal