Lipid lateral diffusion in multilamellar stacked bilayers and in monolayers at the interfaces of air/water (A/W) and oil/water (O/W) have been examined. The technique for the diffusion measurement is fluorescence recovery after photobleaching, in conjunction with the Wilhelmy plate method for equilibrium surface pressure. The oil used is heptane, and the lipid is L-alpha -dilauroylphosphatidylcholine (DLPC). The diffusion in the multi-bilayer system is found to be slower than that in monolayers. at the A/W interface by a factor of 2-3. On the other hand, the diffusion at the O/W interface is established to be constant at lower lipid surface density, while that at the A/W interface is faster in the same range of lipid surface density. The difference at the O/W and A/W interfaces, however, diminishes as the surface density increases and eventually disappears altogether at a high enough surface density that is equivalent to 40 Angstrom (2) per lipid molecule. The observed different diffusion coefficient profile with respect to the lipid surface density is interpreted as the hydrocarbon molecules coming to the surface at the low lipid density, through intermediate stages of interdigitation with lipid hydrocarbon chains, to eventual squeezing out of the monolayer at the high density. Once a fully packed monolayer is formed at the interface, the diffusion is primarily con trolled by the in-plane viscosity and scarcely affected by those of upper and lower phases. The difference in the diffusion coefficients in multi-bilayers and monolayers is tentatively attributed to an artifact of the multiple stacking of bilayers, not to an additional frictional resistance exerted by the apposing hydrocarbon tails within the bilayers.