The present study was carried out on 80 marine scombrid bluefish which were collected from the Red Sea at Suez during the period from January 2000 to January 2001 to investigate callitetrarhynchosis as one of the most prevalent parasitic affections that induces economical losses among the marine scombrid bluefish Euthynnus affinis. The fish were examined externally and necropised. Specimens were collected from the gills. Musculature, peritoneum, myocardium, liver and intestine fixed in 10% neutral buffered formation and 5um thick paraffin sections were prepared, stained and examined microscopically. External examination of naturally infested fish showed no pathognomonic clinical signs, while the internal examination revealed heavy infestation of plerocercoid larvae especially in liver and musculature of the caudal peduncle, branchial cavity as well as other internal organs. The total prevalence was 51.25% ; but, in summer, it was 95%with heavy intensity whichranged from 40 to 70 larvae/fish (summer), 20-34 larvae per fish (sping), 8-12 larvae per fish (autumn), and 6-10 larvae per fish (winter). Parasitological examination revealed that, the obtained larvae were plerocercoids of Callitet-rarhynchus gracilis, which belonged to order Tetrarhynchidea. The gills, musculature. Peritoneum and some internal organs showed pressure atrophy, degeneration and focal necrosis beside some circulatory disturbances and inflammatory reactions with proliferation of melanomacrophages.