The exhaust aerosol of two aircraft at cruise was extensively characterized in the size range from 0.003 to 2 μm for plume ages ≤2 s. The black carbon (BC) exhaust aerosol of an older technology engine (Rolls-Royce/Snecma M45H Mk501) consisted of a primary BC mode with a modal diameter of 0.035 μm and a mode of coagulated BC particles with a peak near 0.15--0.16 μm in diameter. The total number density at the nozzle exit plane was 3¿107 cm-3. In contrast, a modern technology engine (CFM International CFM56-3B1) emitted far smaller BC particles with a primary mode at 0.025 μm and a coagulated mode at 0.15 μm, as well as fewer particles by number with a concentration of 9¿106 cm-3. The single-scattering albedo of the jet exhaust aerosol was 0.035¿0.02 inside the plume, indicating a dominant contribution of ultrafine (D<0.1 μm) BC particles to light extinction. Black carbon number emission indices EI(N) varied from 3.5¿1014 (CFM56-3B1) to 1.7¿1015 kg-1 (M45H Mk501) with corresponding mass emission indices EI(BC) of 0.011 and 0.1 g kg-1. Previously reported corresponding values for a CF6-80C2A2 engine were 6¿1014 kg-1 and 0.023 g kg-1, respectively. A comparison between EI(BC) values calculated by a new correlation method and measured data shows an excellent agreement, with deviations <10% at cruise conditions. By extending the EI(BC) calculation method to a globally operating aircraft fleet, a fleet-averaged emission index EI(BC)=0.038 g kg-1 is calculated. ¿ 1999 American Geophysical Union