Boring patterns of two marine Cyanophytes, Maaigocoleus testarum Lagerheim and Kyrtuthrix dalmatica Ercegovid, which actively penetrate carbonate substrates, have been compared. They show significant differences: 1. The course of penetration in Mastigocoleus is arched to semicircular, in Kyrtuthrix it is predominantly perpendicular to the surface. 2. Each boring tunnel in Mastigocoleus contains only one single trichome, in Kyrtuthrix it contains a looped trichome. 3. The tunnels are circular in cross section in Mastigocoteus and elongated in Kyrtuthrix.