Bryophytes may be more appropriate than vascular plants for testing hypotheses relating to plant reproductive biology due to their relatively simple structure. Nevertheless, they have rarely been used for this purpose, and remain poorly known with respect to reproductive biology. It is, for instance, still not known if bryophytes generally exhibit reproductive costs. We addressed the cost of producing reproductive structures in a bryophyte species, the clonal moss Hylocomium splendens, by following a SE Norwegian boreal spruce forest population in which sporophytic female mature segments grew intermingled with segments devoid of sporophytes for five years. By comparing segment sub-populations with and without sporophytes, respectively, we demonstrated that production of sporophytes had significant costs in terms of less favorable size development of daughter segments, lower branching frequency, and higher risk of not producing new annual segments. From the observed patterns of size development and branching, we identify the late phase of sporophyte development (when the capsule expands and ripens, and spores are produced) as the most expensive developmental stage in terms of demands for resources from the source gametophyte segment. Although costs of sexual reproduction have been demonstrated with certainty only for a few bryophytes, it may well turn out to be easier to demonstrate for bryophyte species than for vascular plant species.