Populations of a modern, stromatolite-building microorganism have been studied in the subtropical Atlantic and the Caribbean area. In four geographically distinct populations, growing under different ecological conditions (exposed to protected, subtidal to intertidal) the same nocti-diurnal rhythm in filament orientation and growth dynamics was maintained. This resulted in the formation of characteristic finely laminated stromatolitic domes. High degree of similarity and overlap in morphometric properties leads to the conclusion that all populations studied belong to the same microbial species. An historic confusion concerning the taxonomic identity of this microorganism has now been resolved and its valid name established: Phormidium hendersonii Howe 1918 (Oscillatoriaceae, Cyanophyta). Stromatolite morphology in this case is determined by the biological (genetic) properties of the organism irrespective of differences in environmental conditions within the range of its distribution. This conclusion bears relevance to the interpretation of ancient stromatolitic structures.