The use of formaldehyde‐killed single‐cell organisms as scattering particles for laser Doppler velocimetry studies in water and salt solutions is discussed. Advantages over traditional scattering particles include ready availability in large quantities, uniformity in size, monodispersity, and the ability to stay suspended in solution for several days. The tracers are colloidal sols for a wide range of densities of aqueous solutions. The microorganisms can be easily stained with a large variety of fluorescent and nonfluorescent dyes before they are used as tracer particles.