Dispersion of neutrally buoyant and buoyant particles in Langmuir circulation is investigated using a numerical model. The crosswind diffusivity increases with decreasing Langmuir number (La) and approaches a constant at small La limit (i.e., high Reynolds number). Although Langmuir cells are more energetic at smaller values of La, the cells are less persistent: small cells are frequently regenerated and later merge with the existing ones. There is a stronger tendency for particles to move within individual cells, but this effect is offset by shorter lifetimes of the cells and frequent releases of particles for moving across the cells. The model results suggest a parameterization of the crosswind diffusivity that depends on the wind stress and Stokes drift current but is independent of the eddy viscosity. Buoyancy reduces the crosswind diffusivity, even for particles whose buoyancy rise speeds are significantly smaller than the downwelling velocity in Langmuir circulation. ¿ 1999 American Geophysical Union