Fifty specimens each of Pulleniatina primalis and Neogloboquadrina acostaensis were selected from 16 late Miocene to early Pliocene samples from DSDP Site 214 in the equatorial Indian Ocean. All specimens were mounted in the umbilical view and their shape outlines automatically digitized using a video scanning system. The shape of each test is represented by a series of 20 harmonic amplitudes, each representing a fixed shape summed to recreate the original test shape. A comparison of shape trends with an oxygen isotope record indicates that environmentally induced morphologic change is more pronounced in the ancestral stock, Neogloboquadrina. These ecophenotypic shape changes appear within N. acostaensis assemblages in both harmonics 2 and 3. In contrast, shape changes in P. primalis appear to be unrelated to the delta 18 O record. These shape changes are believed to be primarily a reflection of genetic factors. The results of this study show that Fourier shape analysis, when integrated with other criteria related to phylogenetic relationships, provides a valuable method for identifying lineages in planktonic foraminifera. Furthermore, the technique is sensitive to shape changes which are attributable to both environmental and evolutionary factors.--Modified journal abstract.