Tetrodotoxin (TTX, <b>1</b>) is a potent voltage-gated sodium channel blocker detected in certain marine and terrestrial organisms. We report here a new TTX analogue, 9-<i>epi</i>TTX (<b>2</b>), and a TTX-related compound, Tb-242B (<b>4</b>), isolated from the pufferfish <i>Takifugu flavipterus</i> and <i>Dichotomyctere ocellatus</i>, respectively. NMR analysis suggested that <b>2</b> exists as a mixture of hemilactal and 10,8-lactone forms, whereas other reported TTX analogues are commonly present as an equilibrium mixture of hemilactal and 10,7-lactone forms. Compound <b>2</b> and TTX were confirmed not to convert to each other by incubation under neutral and acidic conditions at 37 °C for 24 h. Compound <b>4</b> was identified as the 9-epimer of Tb-242A (<b>3</b>), previously reported as a possible biosynthetic precursor of TTX. Compound <b>4</b> was partially converted to <b>3</b> by incubation in a neutral buffer at 37 °C for 7 days, whereas <b>3</b> was not converted to <b>4</b> under this condition. Compound <b>2</b> was detected in several TTX-containing marine animals and a newt. Mice injected with 600 ng of <b>2</b> by intraperitoneal injection did not show any adverse symptoms, suggesting that the C-9 configuration in TTX is critical for its biological activity. Based on the structures, <b>2</b> and <b>4</b> were predicted to be shunt products for TTX biosynthesis.