The effect of early freshwater diagenesis on Mg distributions in former Mg calcite cements was determined by electron microprobe analysis of two major groups of samples: 1) Mg calcites in Recent limestones not yet exposed to freshwater, and 2) Tertiary-Recent Mg calcites and recrystallized equivalents which have undergone freshwater exposure. Pristine Mg calcites are characterized by a uniform Mg concentration. In contrast, former Mg calcites from the Miocene of Enewetak Atoll, most of which are now radiaxial, prismatic calcites, have a much greater variation in Mg content, and traverses across the calcites reveal definite trends. Mg content of the calcite is inversely correlated with abundance of inclusions. Some of the cloudy growth layers are due to infestation by endolithic organisms (fungi?). The small variation in Mg content in original Mg calcite cements is a valuable starting point to be aware of when interpreting Mg trends in recrystallized equivalents. Mg content can be related to inclusion abundance in recrystallized, former Mg calcites, but not in pristine Mg calcites. Therefore, Mg trends in altered materials most likely reflect dissolution/reprecipitation processes, not original Mg trends. A likely cause for the low Mg content associated with inclusion-rich growth layers is relatively high rates of dissolution and reprecipitation in these relatively permeable layers. In some cases, the increased permeability is the result of infestation by endoliths. Such organisms may cause the cloudiness observed in some other radiaxial calcites.