Field tests were made off California to compare estimates of seabird density and species composition resulting from simultaneous ship and aerial observations. In controlled survey experiments, significantly higher mean densities were calculated from aerial observations than from the ship. Under variable field conditions, however, densities derived from vessel surveys for five species groups were statistically indistinguishable from corresponding aerial figures. On ship surveys, 95-97% of all birds were identified to species, whereas from the air, specific identifications were made for 77-96% of the birds seen. About the same total numbers of species were noted from the two platforms, and reported species composition was similar.