Abstract The beta-emitting isotopes Pu 246 and Am 246 were found in the debris of a thermonuclear test. These isotopes have half-lives of 11·2 ± 0·2 days and 25 ± 0·2 minutes, respectively. The chemical evidence for the existence of these isotopes is described. The final mass assignment was made on the basis of mass spectrographic evidence. The beta and gamma energies were obtained from scintillation spectrometer studies. The total disintegration energy of Am 246 was found to be 2·29 MeV, which is in fair agreement with that estimated by Seaborg from closed decay cycle considerations.