With its thick, glassy cortex and wide open, coalescing primary and supplementary apertures Sphaeroidinella dehiscens is a conspicuous component of low-latitude planktonic foraminifer faunas. Sphaeroidinella evolved from Sphaeroidinellopsis through the development of a supplementary aperture on the spiral slde of the test. The Sphaeroidinellopsis-Sphaeroidinella lineage is important in tropical late Neogene stratgraphy. Changes in the proportion of specimens equipped with a supplementary aperture and the size of this aperture have been analyzed in terminal Miocene through basal Pleistocene sections from DSDP sites 502A from the Caribbean Sea, 503 from the eastern equatorial Pacific Ocean and 214 from the equatorial Indian Ocean. Contrary to previous suggestions, the development of a supplementary aperture (the Sphaeroidinella morphotype), marking the base of the basal Pliocene Zone N19 (5.1 Ma) in the standard tropical planktonic foraminifer zonation, took place already in the terminal Miocene (before 5.5 Ma). Following the evolutionary appearance of a supplementary aperture, the proportion in the populations of such an aperture remained low (10-30%) in the earlier parts of the Pliocene. The proportion increased abruptly within a short interval (<0.1 m.y.) in the middle parts of the Pliocene (about 3.4-3.2 Ma), and after 3.0 Ma more than 85% of the specimens had a supplementary aperture. Specimens without a supplementary aperture (the Sphaeroidinellopsis morphotype) persisted all through the Pliocene. However; after 3.0 Ma they represented the smallest specimens in particular populations. During the terminal Miocene and earlier Pliocene the supplementary aperture was constantly very small (<2.62.10 (super 3) mu m (super 2) ). In the middle Pliocene (3.4-3.3 Ma) it rapidly increased in size within a short interval of time (<0.09 m.y.) to post-transitional values of 4.25-22.20.10 (super 3) mu m (super 2) (with one exception). The portion of the spiral test area covered by a supplementary aperture increased from <1% in the earlier Pliocene to 2.80-7.46% in the later Pliocene, marking a 3-8-fold increase in the relative size of the supplementary aperture from the earlier to later Pliocene. Hence, the differences in size of the supplementary aperture were not related to changes in the mean test size. Consequently, the transitional event in the middle Pliocene (3.3-3.4 Ma) reflects the onset of a true evolutionary innovation within the Sphaeroidinella lineage, most probably representing an example of punctuated anagenesis.