Oceanic Anoxic Event 1a (OAE1a) is now generally recognized to correlate with the lower part of the Leupoldina cabri planktonic foraminifer Zone. Unfortunately, the calibration of this zone against the standard ammonite scale has remained uncertain. Our recent high-resolution geochemical study (Carbon-isotope) of Lower Aptian sequences in the Iberian Chain of eastern Spain (Moreno-Bedmar et al., in prep) was used as proxy to further characterize OAE 1a. The sequence provides additional ammonite biostratigraphic data that permit correlation of the ammonite zonation with the isotopic signature of OAE1a. That interval includes specimens we identified with affinity for species of Roloboceras and Megatyloceras in the same beds that contain species of Deshayesites forbesi Casey, and Deshayesites gr. euglyphus/spathi, which are characteristic of the Deshayesites weissi Zone. Our results also indicate that D. deshayesi (d'Orbigny), the nominate taxon which marks the base of the superjacent Zone, first occurs a few meters above the geochemical signature corresponding to OAE 1a. Our isotopic data correlated with the ammonite occurrences are in further agreement with Roloboceras beds in England (Casey, 1961a, b; Casey et al., 1998) that are correlational with the Boreal Deshayesites forbesi Zone, which is coeval with the Mediterranean Deshayesites weissi Zone. Similarly, in the Cassis-La Bédoule area (SE France) the OAE 1a interval also corresponds to the Roloboceras beds, but they have been assigned to the Deshayesites deshayesi biozone (Ropolo et al., 2000; 2006). Here we argue that specimens attributed to D. deshayesi (d'Orbigny) and D. dechyi (Papp) from the Roloboceras levels (Ropolo et al., 2006) can be reinterpreted as belonging to D. forbesi, characteristic species of Deshayesites weissi Zone. Stratigraphic data from Roloboceras beds in le Teil region (Ardech patform, SE France) also reveal the presence of Deshayesites consobrinus (d'Orbigny) and Deshayesites gr. euglyphus Casey at these levels (Moreno et al., 2007). Thus, these taxa further support the stratigraphic position of these beds into the Deshayesites weissi Zone. Our results imply that OAE1 correlative with the Roloboceras beds in southeastern Spain and at Cassis correspond to the D. weissi Zone, as formerly established (Casey, 1961a, b; Casey et al., 1998; Moreno and Bover, 2007; Moreno et al., 2007). R. Casey, Palaeontology 3, 487-621 (1961a) R. Casey, Palaeontographical Society, London, 119-216 (1961b) R. Casey et al., Cretaceous Research 19, 511-535 (1998) J.A.Moreno and T. Bover, in XXIII Jornadas Sociedad Española de Paleontología, Libro de resúmenes. J. C. Braga, A. Checa and M. Company, Eds (Instituto Geológico y Minero de España y Universidad de Granada (2007) pp. 151-152 J. A. Moreno et al., Geogaceta 42, 75-78 (2007) P. Ropolo et al., Géologie Méditerranéenne 25 (1998), 167-175 (2000)