The tolerance of larva at each development stage to organotin antifoulants was studied using Blanus amphitrite DARWIN. The tolerance of nauplius to organotin compounds increased with the larval development. The tolerance of cypris larva was smaller than that of the sixth stage of nauplius larva. These results suggest that the nauplius and the cypris can not be regarded as identical, in physiolgical terms. It has been reminded that the nauplius at the same stage can be replaced by the cypris when comparing the potency of the same compounds, but this substitution is not always possible in the screening test of new antifoulants.