Comparing the degree of openness of different national economies or the variations of openness during different periods is a difficult task. Using the historical data existing for France since the beginning of the 19 th century, we try to show how an accurate and consistent picture of the gradual internationalisation of the French economy can be drawn from this set of indicators. The historical outlook we tried to establish for France may provide a new insight into the original features of the recent times, since the 1970s. The internationalisation process is not of course something new in itself. But, according to all available quantitative indicators, it is now more continuous and more intense than ever before. The coefficients measuring specifically the degree of openness of the manufacturing sector show the most significant progression, now surpassing by far the previous exceptionally high peaks reached during the 1920s – and this in both directions : for the growing share of imported manufactures in the French market goes together with the unchallenged pre-eminence of the manufacturing sector among French export trades. This new conjunction is relevant to explain the most negative characteristic of the present situation : for the first time, an acceleration in the process of opening is accompanied by a protracted weakening of the French (and European) economic growth.