A new laser particle sizing instrument has been constructed for on-line particle sizing of combustion exhausts. The instrument measures the optical extinction associated with single particles as they flow through focused laser beams. Two focused beams which cross at their minimum diameter along with coincident pulse detection electronics are used to size only those particles which traverse simultaneously both focused beams. Donut mode intensity beams together with double pulse detection circuitry are used to ensure the particles which are measured have traversed the peak laser intensity. Opaque particles in the size range 0.05 μm to 5.0 μm diameter can be sized with a diameter resolution of better than 10%. The present instrument can size ∼1000 particles in 3 minutes at its maximum allowable particle concentration of 105 particles/cm3. Both particle size distribution and particle emission rate from a diesel automobile exhaust were measured over an FTP cycle.