An image analysis system was used to measure scanning electron microscope images of Late Miocene reticulofenestrid nannofossils (approximately 7-8 Ma) from Deep Sea Drilling Project Sites 608 (north Atlantic Ocean) and 251 (equatorial Indian Ocean) and Ocean Drilling Program Sites 714 (equatorial Indian Ocean) and 852 (eastern equatorial Pacific Ocean). Morphometric parameters including length, width, area, and ellipticity for the outer shield, inner shield, central tube and central area were measured for over 3000 specimens. A dramatic size decrease event with important evolutionary implications for the reticulofenestrid group was found in three sites. This work, combined with two other studies, makes this one of the best documented size decrease events. Measurement of central area ellipticity suggests that traditional methods for dividing Late Miocene reticulofenestrid species are not morphometrically valid. Fine fraction oxygen isotopes may indicate a cold-water trend at Site 608 during the event.