The standard B-chromosomes ofListera ovataare euchromatic, sub-metacentric and about half the size of the smallest autosomes. They are widespread in all populations throughout Europe. There are two derivative B-chromosomes, one sub-terminal type and a small telocentric type which are found sporadically in most populations. In Britain, the percentage of B-chromosomes per population varies from 89.12% to 62.82%. Populations found in wet habitats have about 14% more B-chromosomes than those found in normal habitats. There is a latitudinal bias in the percentage of B-chromosomes per population which range from an average of 21.81% for Central Italy, 42.85% for Switzerland, about 50% for Holland and 73.93% for Britain.