Penetration of calcareous substrates by algae involves different micro environments. Endolithic algae include two different life forms: (1) algae colonizing existing spaces within the rock, and (2) algae aetively boring within the carbonate substrate. Two species of boring Cyanophyta were studied, and their borings were compared in exposed calcite (Island spar). The size and pattern of borings reflect the corresponding properties of the algae and show taxonomic distinctions. The direction of the tunnels and their wall sculpture are determined by the planes of crystal cleavage and twinning. Algal boring is a dissolution process performed by the terminal cells of endolithic filaments. The space dissolved away by an alga has the shape of a miniature calcite crystal. Dissolution proceeds along the crystal twinning lines diagonally to the previous microcrystal space. The tunnel represents a sequence of such microcrystal-shaped spaces. Both biological and mineralogical determinants should be considered in the interpretation of algal boring patterns.