estructuring the classification of Cretaceous age planktonic foraminifer requires studying morphological changes through time and phylogenetic relationships between species. In the following study, Gublerina cuvillieri and its proposed ancestor Heterohelix rajagopalani were picked from Ocean Drilling Program Hole 761B (Exmouth Plateau), and their relationship was evaluated using biometric analyses and morphological observations from SEM images. The phylogenetic relationship between G. cuvillieri and H. rajagopalani has been the subject of some debate. Gublerina cuvillieri was first described by Kikoine (1948) from southern France, and Govindan (1972) described Gublerina rajagopalani from southern India. Nederbragt (1991) has since suggested that G. rajagopalani actually belongs in another genus, Heterohelix. In this study, quantitative data from x-ray image measurements provide evidence that H. rajagopalani is the direct ancestor of G. cuvillieri. The measurement data for the two species are virtually identical for the biserial portion of the tests, indicating a close phylogenetic relationship. Heterohelix rajagopalani and G. cuvillieri diverge later in their ontogenetic history as expressed in G. cuvillieri by separation of the serial chambers, development of a multiserial stage, greater chamber number, and concentration of ornamentation elements in bands extending around the chamber edge. Isotope data indicates the two species lived at relatively the same water depth. This study is the beginning of a reevaluation of the gublerinid phylogenetic relationships.