Three cores taken in the tidal flat of the Bay of Mont Saint-Michel were dated with two different methods, one involving use of the naturally-occurring isotope super(210)Pb, the other using a man-made super(137)Cs isotope. For two of the cores it was possible to calculate sedimentation rates. In these cores the data obtained with the two different methods agreed well: sedimentation rates of 0.39 plus or minus 0.08 ( super(210)Pb method) resp. 0.48 cm/y ( super(137)Cs method) were found in one core; and of 0.47 plus or minus 0.07 and 0.40 cm/y in the other. In the third core no reliable sedimentation rate could be calculated, although the Cs data indicate that the sedimentation rate is in the order of 5 cm per year. These data are in good agreement with sedimentation estimates based on cartographical data and comparison of successive aerial photographic surveys.