We study low-temperature nucleation in kinetic Ising models by analytical and simulational methods, confirming the general result for the average metastable lifetime, τ = A exp(βΓ) (β = 1/kBT ) [E. Jordão Neves and R. H. Schonmann, Commun. Math. Phys. 137, 209 (1991)]. Contrary to common belief, we find that both A and Γ depend significantly on the stochastic dynamic. In particular, for a "soft" dynamic, in which the effects of the interactions and the applied field factorize in the transition rates, Γ does not simply equal the energy barrier against nucleation, as it does for the standard Glauber dynamic, which does not have this factorization property. [7], and atmospheric science [8], to mention just a few. However, many questions in nucleation theory are still unresolved, and recently there has been much interest in kinetic Ising systems as models for nucleation. In particular, much work has been done on their dynamical behavior at very low temperatures [9,10,11,12,13,14], where it is influenced by lattice discreteness. It is then possible to calculate exactly both the shape of the critical nucleus (the saddle-point configuration) and the most probable path during a nucleation event. In a typical numerical experiment, the system is prepared in a metastable state with all spins positive in a negative applied field. During each MC step (MCS), a randomly chosen spin is flipped with a configuration-dependent transition rate W that satisfies detailed balance, so that it will drive the system to thermodynamic equilibrium. The metastable lifetime is measured as the average number of MCS until the magnetization reaches zero. In the regime of singledroplet decay studied here, the lifetime measured in MCS is independent of the system size [9,11,15]. In the lowtemperature limit the lifetime has been rigorously shown to be [9] τ = Ae βΓ .(Here the only dependence on the temperature T is through β = 1/k B T , where k B is Boltzmann's constant (hereafter set equal to one). It is often assumed that Γ equals the energy difference between the saddle point and the metastable state [13,14], independent of the specific stochastic dynamic. In this Letter we show that this is not always so. In particular, we describe two dynamics that both obey detailed balance but have different values of Γ and A for all values of the applied field, despite having the same saddle-point configuration.At sufficiently low T , the saddle point was shown in Ref.[9] to be an ℓ × (ℓ − 1) rectangle of overturned spins with a "knob" of one overturned spin on one of its long sides. The critical length ℓ = ⌊2J/|H|⌋ + 1 for all |H| ∈ (0, 4), where ⌊x⌋ is the integer part of x. Here, J > 0 is the nearest-neighbor interaction constant of the Ising model, which will henceforth be set to unity. The critical length thus changes discontinuously at values of |H| such that 2/|H| is an integer.The square-lattice S = 1/2 Ising ferromagnet with unit interaction is defined by the Hamiltonian H = − α,β σ α σ β − H α σ α , where the Ising spins σ α = ±1, H is the applie...