Chemical determinations of Mn on 147 samples of carbonate rocks from Mesozoic Tuscan sequence (Rhaetian to Neocomian) of Lima Valley (Northern Apennines) have been carried out. From the relationships between Mn content on one hand, the mineralogy and depositional environments on the other, the following conclusions can be drawn: (1) Mn appears to be mainly associated with the carbonate fraction. (2) The low Mn content of the shallow water formations reflects the original carbonate phase, mainly aragonite; sharply higher Mn content has been found in deeper sea formations, in origin essentially calcitic. (3) Diagenetic processes, particularly dolomitization, do not appear to have seriously modified the original Mn content. (4) The relatively high Mn content in some stratigraphic levels could be related to periods of intensive weathering and/or to a larger availability of Mn (super 2+) , due to more reducing sea water conditions.