Previous morphological research of the Myosotis sylvatica group in the Department of Biology has shown there is another taxon present in Slovenia. The majority of the results of the measurements have placed the new taxon between the M. sylvatica and the M. decumbens. The only reliable trait between taxa is pollen size. Since pollen grain size indicates genome size we measure nuclear genome size of the representatives from the Myosotis sylvatica group from the territory of Slovenia and the neighbouring areas and count their chromosome numbers. The research includes 50 plants from Slovenia, Croatia, Austria and Italy. DNA image cytometry was used to measure nuclear genome size in the root tips. The following karyotypes were obtained: 2C = 1,21 pg for M. decumbens, 2C = 1,13 pg DNA for the new taxon and 2C = 0,62 pg DNA for M. sylvatica. Data about nuclear DNA amount was compared to the past measurements of pollen sizes. We found a positive correlation between these traits. Determining the chromosome number was less successful. Despite this, it is apparent that the new taxon has a value of chromosome number between M. sylvatica and M. decumbens.