The neonatal line (NNL) has been interpreted as broadly Retzius line, which can be found in the enamel of all deciduous teeth and of all first lower permanent molars. Since the NNL is discussed as locus minoris resistentiae during the initiation of early child-hood caries (Nursing-Bottle-Syndrome), this study was aimed to investigate the ultra-structure with a scanning electron micro-scope. The study comprised five sound primary front teeth and five sound primary molars. All teeth were hemi-sectioned (MIKRO-TRENN MT 1557, Hofer, Swiss). Afterwards the smear layer was removed by acid etching with 35% phosphoric acid for 30 s. The examination was carried out under a SEM 515 (Philips, Eindhoven, The Netherlands) using a constant cathode voltage of 20 keV and a maximum magnification of 20.000. In seven teeth a NNL was visualized by SEM. At 1.000x magnification only an irregular structure of the enamel prisms was observed. Detailed images showed a typical interruption of the prisms at the NNL between 5.000x to 20.000x magnification; nearly all crystallites of a prism were not mineralised in a length of 0,5 µm. As a result of frequent consumption of erosive and/or cariogenic drinks and foods the predominant demineralisation of the enamel nearby the NNL could be enabled. This gives an explanation for initiation and the rapid progression of the carious process at the NNL. Schlagwörter: Neonatallinie, Ultrastruktur, REM