The biostratigraphically important Ammonitina of the Chacay Melehue key section situated in Neuquen Province (Argentina) are taxonomically revised and illustrated. The classical section of Caracoles (Antofagasta Province, Chile) is also reviewed. New taxa are: Stebnocephalites n. gen., (type species: Indocepbalites gerthi Spath), Xenocephalites stipanicici n. sp. (=“X.berrerodouclouxi” Stipanicic, nomen nudum) and Oxycerites (Alcidellus) obsoletoides n. sp. A succession of seven biostratigraphic units is identified,spanning the interval from the early Upper Bajocian (#1) up to the Middle Callovian (#7), capped by the gypsum of the Tabanos Formation. Four of these assemblage zones (#3–6) can be very widely recognized, from Mexico to the southern Andes. They are therefore used to define a provincial standard chronozonation for the Andean Upper Bathonian Lower Callovian, although the primary Bathonian Callovian boundary cannot be precisely located. The zonation is as follow: 1:(?) Megasphaeroceras rolundum Assemblage Zone, Lower Upper Bajocian; 2: Cadomites-Tulitidae mixed assemblage, (? Lower-) Middle and Upper Bathonian; 3: steinmanni Zone, index Lilloettia steinmanni ( Spath). Uppermost Bathonian, with two local horizons: Stebnocepbalites gertbi horizon (Argentina) and Cboffatia jupiter horizon (North Chile); 4: Vergarensis Zone, index: Eurycephalites vergrensis ( Burckh.), near Bathonian Callovian boundary; 5: Bodenbenderi Zone, index: Neuqueniceras (Frickites) bondenbenderi ( Tornq.), Lower Callovian; 6: Proximum Zone, index: Hecticoceras proximum Elmi, uppermost Lower Callovian; 7: Reineckeia assemblage, Middle Callovian.