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Tests were conducted aboard pelagic longline vessels fishing off the coast of Uruguay, to determine the effect of hook type (circle hook vs J-hook) on the catch and length composition of target and non-target species. Both American and Spanish style longlines were used during these experiments. American style longlines consist of a 3.6 mm polyamide monofilament mainline with polyamide or polyester buoys lines 9-36 m long. Spanish style longlines consist of 5-mm polyethlene multifilament mainline with buoy lines (similar in thickness and material to mainline) 12-18 m long. On the American style longlines, albacore tuna and shortfin mako shark catches were significantly higher on circle vs J-hooks. Catches of other tunas and sharks (blue and porbeagle) were also higher on circle hooks, but were not significantly higher. There was no significant difference in the catches of seabirds or sea turtles between the two types of hooks on the American style longlines. On the Spanish style longlines, albacore tuna catches were also significantly higher on circle compared to J-hooks, and there was a significant decrease in pelagic stingray catches on circle hooks. No significant difference in the catch rates of seabirds or sea turtles was seen between the two types of hooks on this type of longline. There was no significant difference in the length distribution for any species between the two hook types on either type of longline. There were also no significant differences between hook types for any hooking location of incidentally caught sea turtles.