We evaluated the regional distributions of six nitrogen (N sub(2))-fixing bacteria in the North Pacific Ocean using quantitative polymerase chain reaction amplification of planktonic nifH genes. Samples were collected on four oceanographic research cruises between March 2002 and May 2005 that spanned a latitudinal range from 12 and 54 between 152 and 170. Samples were collected throughout the upper ocean (<200 m) in the northern regions of the South Pacific Subtropical Gyre (SPSG), equatorial waters, the North Pacific Subtropical Gyre (NPSG), the North Pacific Transitional Zone (NPTZ), and within the Pacific Sub Arctic Gyre (PSAG). There were distinct spatial gradients in concentrations of nutrients, chlorophyll, and the abundances of N sub(2)-fixing bacteria within the various oceanic biomes. In general, nifH-containing bacteria were most abundant in the midregions of the NPSG (latitudes between 614 and 29), where unicellular cyanobacterial phylotypes dominated nifH gene abundances. The abundances of all nifH-containing groups declined within the northern and southern regions of NPSG. Although nifH-containing groups were detectable in the northern regions of the SPSG, throughout the equatorial waters, and within the NPTZ, gene copy abundances of most groups were lower in these regions than those found the in the NPSG. In the NPSG, surface water abundances of the various nifH phylotypes examined ranged from <50 copies L super(-1) to 610 super(5) nifH copies LS super(-1). Overall, the abundances of an uncultivated, presumed unicellular nifH sequence-type (termed Group A) were the most abundant and widely distributed of the phylotypes examined. Our results indicate that the distributions of N sub(2)-fixing plankton were largely restricted to the subtropical regions of the North and South Pacific Oceans.