During the taxonomic analysis of strains from marine phytoplankton samples, it was found that a sample contained isolated or short chains of 2-4 small cells. Observations under the electron microscopes revealed the presence of Extubocellulus spinifer (Hargraves & Guillard) Hasle, Stosch & Syvertsen, a minute species until now not reported for the Southeastern Pacific Ocean. In this paper we describe the characteristics of the Chilean cells, comment on the variability of some of its morphological features, and discuss about the differences between the two known species of the genus.