SUMMARYInitial results of investigation of the diagenesis of calcareous algae by electron microprobe are:1A few conceptacles and other cavities in Recent algae specimens are partly to completely filled with younger carbonate. In contrast to the surrounding skeleton of about 10–20% mol MgCO3‐calcite, these deposits are either very low (0–2%) or very high (15–50%) in MgCO3content.2Highest magnesian values in Recent algae studied are thought to be in high‐Mg calcites rather than in any other phase, with very few possible exceptions.3Calcian dolomite replacing fossil algae in cores from Eniwetok Atoll commonly has Ca‐rich lenses resembling exsolut