A model of ice crystal nucleation and growth in a diluting aircraft exhaust plume is used to evaluate the physical processes responsible for the ice number densities and size distributions measured on May 4, 1996. Predicted ice crystal size distributions are compared with observations at about 70 seconds plume age. The simulated crystals grow to about 2 &mgr;m radius within 2--3 seconds, then remain approximately constant in size until 30--60 seconds plume age when crystal sublimations begins. The calculated crystal size distribution has approximately the same volume mode radius as the observed size distribution (1--2 &mgr;m); however, the model does not predict as many crystals larger than 2 &mgr;m radius as indicated by the measurements. Due to the low ambient temperature (-61 ¿C), large ice supersaturations are generated in the plume, and all exhaust soot particles and ambient aerosols entrained into the plume before about 1 second plume age are activated to ice nuclei. The simulations indicate that if the soot particle emission index is on the order of 2¿1012 particles (kg fuel)-1 or lower and freshly nucleated sulfate aerosols are too small to freeze in the young plume, then the majority of the contrail ice crystals may be nucleated on ambient aerosols entrained into the exhaust plume. ¿ 1998 American Geophysical Union