The Agrio Formation of the Neuquén Basin, Argentina, contains an extensive sequence of ammonite faunas, most of which are monogeneric. Detailed collecting through 15 sections across the basin has facilitated a major revision of the ammonite zonation. Formerly embracing four broad zones, the Agrio Formation is now divided into nine zones, the lowest four of which are divided into a total of 11 subzones. The new zonation provides a standard against which other South American faunas can be compared. The degree of subdivision now achieved is comparable to that for the ‘standard’ sequences of the West Mediterranean region. Although it is not possible to correlate the two regions in detail, the occurrence of some widely distributed genera (Olcostephanus, Karakaschiceras, Oosterella, Spitidiscus and Crioceratites) at well-defined levels in the Neuquén Basin provides some crucial links. Thus the approximate positions of the Lower/Upper Valanginian, Valanginian/Hauterivian and Lower/Upper Hauterivian boundaries can be determined.