A portable Gardner counter on board the R/V Alpha Helix was used to obtain Aitken nuclei data over the Pacific Ocean from San Diego, California, to San Martin, Peru, during a 2-week period in March 1976. The number concentration distribution reflects the passage through several distinct synoptic scale circulation patterns. The highest (N) concentrations, over 4000 N/cm3, were measured during a short period of time in the northeast trade wind belt off Central America, while the lowest values, near 250 N/cm3, occurred in the southeast trade wind circulation off the Peruvian coast.