We evaluated whether bearing tetrodotoxin (TTX) affects salinity stress in the juvenile tiger puffer Takifugu rubripes. Juveniles of hatchery-reared non-toxic T. rubripes [body weight (BW): 1.7 ± 0.2 g, n = 120] were divided into six tanks and acclimatized to salinity (8.5 ppt) that is equivalent to blood osmolality. Fish in three tanks were fed non-toxic diet, and those in the other three tanks were fed a TTX-containing diet (356 ng/g diet) three times a day until satiation. In each diet treatment, salinity of one tank was kept at 8.5 ppt, and the other two tanks were adjusted to either 1.7 or 34.0 ppt, and fish were reared for another 33 days. Then, we compared survival, growth, TTX accumulation, plasma osmolality, plasma cortisol, and glucose levels among treatments. We detected TTX only in the fish in the TTX-diet groups. Survival was highest at 8.5 ppt (70%) and lowest at 1.7 ppt in the TTX-diet group (20%). The BW was greater at 8.5 ppt, and plasma osmolality was significantly higher at 34.0 ppt than at any other salinities. Plasma cortisol level was significantly higher but glucose level was lower at 1.7 ppt. Possessing TTX at a low salinity may be lethal to tiger puffer juveniles. [ABSTRACT FROM AUTHOR]