We report a molecular dynamics study of homogeneous bubble nucleation in a Lennard-Jones fluid. The rate of bubble nucleation is estimated using forward-flux sampling (FFS). We find that cavitation starts with compact bubbles rather than with ramified structures, as had been suggested by Shen and Debenedetti (J. Chem. Phys. 1999, 111, 358 1). Our estimate of the bubble-nucleation rate is higher than predicted on the basis of classical nucleation theory (CNT). Our simulations show that local temperature fluctuations correlate strongly with subsequent bubble formation; this mechanism is not taken into account in CNT.