Errors associated with subisokinetic sampling were determined for a 1·65-in. dia. filter holder. Data for 3–25 μ dia. particles were obtained for several subisokinetic flow rates including impaction for velocities between 2·7 and 30 m.p.h. For isokinetic flow into nozzles, the deposition of particles in the nozzle was significant and increased with particle size.