Diel variations in beam attenuation profiles in the upper water column were observed during Joint Global Ocean Flux Study (JGOFS) programs in the North Atlantic and Equatorial Pacific. Beam attenuation due to particles in surface waters increased during the day by as much as 70% and decreased at night in both studies. Planktonic processes are probably responsible for these changes. Since many biological and chemical species have strong gradients in the upper 100 m, regular oscillations in the depth of mixing can be an important forcing function for vertical exchange in surface waters. When it occurs, this mixed-layer pump is important in sustaining new primary production and in removing particles from surface waters, and should be included in models of oceanic surface mixing.