Quantitative analyses of the calcareous nannofossil Reticulofenestra asanoi and related species have been performed on the Early-Middle Pleistocene transition in the Mediterranean Sea (ODP Sites 976 and 963) and Atlantic Ocean (DSDP Hole 610A) in order to improve the understanding of their stratigraphic distributions. Abundance patterns have allowed the identification of the lowest common occurrence (LCO) and highest common occurrence (HCO) of R. asanoi in a short interval below and above the lowest occurrence of Gephyrocapsa sp. 3. Correlation with oxygen isotope stratigraphy at Site 976 places the LCO of R. asanoi at the Marine Isotope Stage (MIS)34-33 transition and its HCO at the MIS 23-22 transition. At Site 963, the HCO of R. asanoi (estimated age of 0.96 Ma correlated to MIS 25) is regarded as "artificially" low, and its highest occurrence (estimated age 0.90 Ma correlated to MIS 23) has therefore been used for bio-chronostratigraphic correlation. The LCO of R. asanoi is estimated at 1.05 Ma at Site 963 and 1.17 Ma at Hole 610A, which suggests correlation to MIS 30 and MIS 35, respectively. These data suggest a possible diachrony for the LCO of R. asanoi.