A Bk 247 sample was prepared by intensive helium ion bombardment of a curium sample containing Cm 246. The alpha half-life was determined by a mass-spectrometric isotopic dilution method, using Bk 249 as the diluent. The value obtained was 1380±250 y, considerably shorter than previous estimates. The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk 248 with a half-life greater than 9 y. No growth of Cf 248 was detected, and a lower limit for the β - half-life can be set at about 10 4 y. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 y.