β-delayed fission (βDF) decay of a low-spin (ls) and a high-spin (hs) isomer in 188 Bi was studied at the ISOLDE facility at CERN. Isomer-selective laser ionization and time gating were employed to investigate each isomer separately and their βDF partial half-lives were determined: T 1/2p,βDF (188 Bi hs) = 5.6(8) × 10 3 s and T 1/2p,βDF (188 Bi ls) = 1.7(6) × 10 3 s. This work is the first βDF study of two states in one isotope and allows the spin dependence of low-energy fission to be explored. The fission fragment mass distribution of a daughter nuclide 188 Pb, following the β decay of the high-spin isomer, was deduced and indicates a mixture of symmetric and asymmetric fission modes. Experimental results were compared with self-consistent mean-field calculations based on the finite-range Gogny D1M interaction. To reproduce the measured T 1/2p,βDF (188 Bi hs), the calculated fission barrier of 188 Pb had to be reduced by ≈30%. After this reduction, the measured T 1/2p,βDF (188 Bi ls) was in agreement with calculations for a few possible configurations for 188 Bi ls. Theoretical βDF probabilities for these configurations were found to be lower by a factor of 4-9 than the βDF probability of 188 Bi hs. The fission fragment mass distribution of 188 Pb was compared to the scission-point model SPY and the calculations based on the finite-range liquid-drop model. The first observation of βDF for 190 Bi is also reported.