In situ cloud microphysical measurements are presented for two ship tracks detected off the Washington coast in three successive satellite images from the advanced very high resolution radiometer (AVHRR) channel 3 (3.7 μm). Aerosol and cloud water chemical data suggest that the tracks were produced by effluents from ship stacks. Cloud droplet spectra measured in the ship tracks had effective radii about one half of those measured in the ambient clouds. A drizzle mode was present in the ambient cloud, but this was largely suppressed in the ship tracks. Analysis of cloud condensation nucleus (CCN) and total particle spectra in and around the ship tracks suggest that the stability of the tracks could have been due to a feedback between the lowering of CCN concentrations as the ship plumes diluted, consequent increases in peak supersaturations in the cloud, and a modest amount of particle growth by gas-to-particle conversion. ¿ 1998 American Geophysical Union