The size distribution of particles smaller than 0.5 &mgr;m has been measured over the tropical Atlantic and Pacific oceans with a differential mobility analyzer. In regions remote from continental influences the size distribution generally has peaks at about .02-.03 &mgr;m and at .08-.15 &mgr;m with a minimum in the .05-.08 &mgr;m radius range. The data provides strong evidence that nonprecipitating clouds play an important role in transferring material from the gas phase and from smaller particles into the 0.08 to .15 &mgr;m radius range, and that they are responsible for the doubly peaked size distributions frequently observed over the oceans.