Vertical profiles of Aitken nuclei, cloud condensation nuclei, sulfate, and SO2 concentrations have been measured in the marine atmosphere under relatively clean conditions. The vertical profiles of Aitken nucleus concentrations commonly showed maximum values just above cloud tops. We interpret these measurements as evidence for the nucleation of new sulfate particles in these locations. The cloud condensation nucleus profiles are consistent with this interpretation. A theoretical analysis of the measurements, utilizing a model of homogeneous, heteromolecular nucleation, predicts that the nucleation of new sulfate particles should occur primarily in the proximity of clouds. Implications of these results for dimethyl sulfide-cloud-climate hypothesis are discussed. ¿American Geophysical Union 1990