A comparative investigation into nuclear DNA amounts using flow cytometry and video-based Feulgen densitometry was carried out in 289 accessions of 138 different moss taxa (Bryatae), originating from Austria, Switzerland, Spain, Mexico and the USA. Sampling included species from all major moss clades (except Sphagnum). Flow cytometry data agreed highly with the Feulgen data, which once again demonstrates the high reliability of both methods for DNA amount determination. For the first time, extensive data on absolute C-values of mosses are available. Haploid DNA contents (1C) ranged from 0.174 to 2.16 pg, which represents only about a 12-fold variation. This low C-value variation is remarkable when compared to angiosperms which vary approx. 1000-fold. C-values are usually relatively constant within genera and even families; however, genera with varying C-values also exist. From the low frequency observed, secondary polyploidy plays only a minor role in mosses. Possible reasons for the low C-value variation are discussed.