During a survey of the diatom flora present on an artificial substratum (plexiglass) submerged off the southern Bulgarian Black Sea coast, a small-celled diatom taxon, reported previously from the Black Sea as Amphora (Halamphora) sp. S21, was observed. Detailed light and scanning electron microscope observations showed that the original taxon actually included two different taxa: an unknown Halamphora species described here as H. valdeminutissima sp. nov., and a second taxon, which, based on SEM observations, could be identified as Amphora micrometra Giffen. The morphology of both taxa is described in detail and a comparison with similar taxa is provided. Additionally, environmental data of the habitat is also provided.