The distribution of male, female and sporulating plants of Thamnobryum alopecurum( Hedw.) Nieuwl. [= Thamnium alopecurum (Hedw.) Schimp.] in the Netherlands is given in an attempt to clarify the problem, of why this species sporulates so seldom and in which environment sporophytes are formed most frequently. In places with air with a constantly high humidity the plants bear the largest numbers of inflorescences and in such places female and male plants also grow more strongly intermingled. In places with (periodically) drier air the plants remain mostly sterile or nearly so and develop into large sprouting systems with little contact between each other; even „moss balls” can occur then. Another important point is that in some localities only males or only females occur and in some one sex is very rare. The length of the seta of Thamnobryum alopecurum appears to be rather variable (10-30 mm). though within one specimen remarkably constant: mostly there is only 2-3 mm difference between the longest and the shortest seta per plant. Populations occur containing both plants with long setae and plants with short setae but without intermediate specimens. Elsewhere a continuous range has been found.