Hamatocaulis vernicosus, a rare moss species, was monitored in 33 fens in the Czech Republic for five to six years. Population size, vitality and trends of population development were recorded. Water chemistry, water level fluctuation, vegetation type and cover, as well as mowing regime were assessed and the effect of these potential predictors on the species populations was examined. Populations of H. vernicosus were affected mainly by the density of vascular plants, the species thrived best in habitats with sparse herb and abundant "brown moss" cover. Other important factors included water table fluctuation and water concentration of iron. Populations were more vital and prospered better in sites with a stable water table and more iron-rich conditions. Dependence of population parameters on other measured characteristics of water chemistry was not detected.