An attempt was made to separate unidentified derivatives of tetrodotoxin (TTX) from toxic specimens of a xanthid crab Atergatis fforidus collected at Kojima, Ishigaki Island, Okinawa Prefecture. Seventeen crab specimens (300g) were minced and extracted with acidic ethanol. After being treated with activated charcoal, the toxin extract was purified by Bio-Gel P-2 and Bio-Rex 70. Two unidentified toxins, designated TTX-U 1 (0.9mg) and TTX-U 2 (0.2mg) were separated with TTX (0.6mg). TTX-U 1 and U 2 showed specific toxicities of 3800 and 2200 mouse units per mg on i.p. injection into mice. They gave protonated molecular ion peaks at m/z 336 and 290 on electrospray ionization mass spectrometry. TTX-Ul was converted into TTX, when reduced with NaBH₃CN. ¹H NMR spectral data of TTX-U 1 and U 2 corresponded well with those of 11-oxotetrodotoxin and 11-nortetrodotoxin-6 (R) -ol.