Individual lipid movement in a monolayer is studied over long time intervals (500 s) by darkfield microscopy of single lipids labeled with gold colloids (30 or 100 nm in diameter). Dimyristoyl phosphatidylcholine in the fluid phase shows normal diffusion, with a diffusion coefficient of (1.1 +/- 0.2) x 10(-8) cm(2)/s. Since this is consistent with values derived from the diffusive transport of many Lipids, the analysis of gold-tagged lipids in a monolayer provides a reliable picture of lipid diffusion on the level of single molecules.