Migrating cells originating selectively in the ventral lateral edge of the somites adjacent to the mouse hindlimb bud have been studied in transverse sections by transmission and scanning electron microscopy. Collected during the 10th and 11th gestational days, the embryos have been classified according to the number of metameres. As soon as the 28 somite stage, discrete cytological modifications occur in a limited caudal area of the ventro-lateral somitic edge. Loosing the typical epithelial arrangement characteristic of the dermatome cells, these ventral cells show large areas of close contact between their plasma membrane and a superficial microfilamentous material accumulates in the contact areas. At the 33 somite stage, the same groups of cells elongate and form long cellular trails invading the proximal area of the limb bud mesoderm. The migrating cells become polarized along the migrating axis and they retain large and smooth intercellular contacts with each other. Very selective ultrastructural features of the migrating somitic cells can be interpreted in relation to their cinetic activity or to their early myogenic differentiation. In addition to their mutual superficial relationships, the migrating cells are characterized by the presence of numerous oriented microtubules, of a high number of active mitochondria, of an abundant granular endoplasmic reticulum and of an hypertrophied Golgi apparatus regularly located near the nucleus in the "trailing" edge of the cells. Several dense granules with a diameter of 8 nm are present in the mitochondrial matrix. The extensive Golgi apparatus is associated to numerous thick walled vesicles, which increase from 60 to 150 nm in diameter as they become closer and closer to the plasma membrane. These vesicles are absent in the mesodermal cells of somatopleural origin; their presence in the migrating somitic cells is probably related to an early myogenic differentiation. The observation made near the distal end of the somitic cellular trails suggests that the more distal somitic cells rapidly loose their ultrastructural particularities as soon as they are dispersed in the limb bud mesoderm; this aspect of the processus, however, requires the study of later developmental stages. Other observations made in the same material bring some precisions to the ecto-mesodermal relationships which are established in the apical area of the limb bud. Scanning electron microscopic observations of thick sections reveal that the outer mesodermal cells of this area send numerous filopodia which make contact with the basement membrane underlying the apical ectodermal ridge.