<p>Genetic differentiation of <em>Calypogeia muelleriana </em>s.l. was studied using isozyme analysis. Two forms of this species: typical and atypical were reported from Poland. The 10 putative loci in 7 enzyme systems were analyzed in 58 samples: 34 of the typical and 15 of atypical form. The isozyme studies revealed that the typical and atypical forms of <em>C. muelleriana </em>in Poland are clearly genetically different. Typical plants morphologically correspond to the type specimen of <em>C. muelleriana</em>, but atypical form is a new, genetically distinct but unrecognized so far taxon. Each group is defined by several fixed alleles present in all populations. The UPGMA dendrogram based on Nei’s genetic distances shows that both taxa (<em>C. muelleriana </em>and the newly detected taxon) clearly differ from <em>C. azurea </em>– the species used as a reference group. Genetic distance among two groups of <em>C. muelleriana</em> (<em>D </em>= 1.093) was almost the same as among <em>C. azurea </em>and the newly detected taxon (<em>D </em>= 1.060). Genetic distance among <em>C. azurea </em>and the typical form of <em>C. muelleriana </em>was the lowest (<em>D </em>= 0.628).</p>