Breast cancer is the leading cause of cancer deaths in women. Electron microscopy allows establishing constitutive characteristics of cells between different cell populations. CD44 high / CD24 low mammary cancer stem cells are a population of intratumoral cells associated with chemoresistance and metastasis, whose ultrastructure has not yet been well studied. The objective of this work was to know the ultrastructural characteristics of cells with cancer stem cell phenotype, of the triple negative mammary tumor cell line MDA- MB-436 436 using scanning and transmission electron microscopy, and to contrast them with adipose-derived mesenchymal stem cells. Previously, by immunomagnetic purification using anti CD44 and anti CD24 antibodies bound to magnetic beads, cells populations was obtained from 10x106 cells of the MDA-MB-436 line, which, adipose-derived mesenchymal stem whereas cultivated on coverslips for scanning electron microscopy; while for transmission electron microscopy a cell pellet was obtained, then fixed with 2.5 % glutaraldehyde and post fixed with OsO4 1 %. For high resolution optical microscopy, toluidine blue was used as staining. After obtaining the phenotype of cancer stem cells, their pluripotency was corroborated by detecting the expression of the Oct4 and nanog genes by RT-PCR. Our results show that the cells of this phenotype are small, rounded, covered by abundant but short microvilli; the cytoplasm has organelles of cellular secretion and abundant elongated mitochondria; the nucleus is eccentric occupying half of the cellular volume, the nucleolus is bulky and the heterochromatin is attached to the inner nuclear membrane. It is concluded that the cellular immunophenotype studied is a sub-cellular population within the line studied that differs in size and ultrastructure of the adipose stem cells.