The morphology of field-collected plants of Acrochaetium densum (Drew) Papenfuss from the Dutch coast is described in some detail. This species is characterized by a persistent septate spore, a multicellular filamentous prostrate part and numerous rather short erect axes, which are secundly branched in mature plants. Reproduction takes place by monospores which are typically formed in rows of two or three successive monosporangia, ripening simultaneously. In culture the plants also formed tetrasporangia and subsequently the life-history could be completed. It appeared to be a diplobiontic cycle consisting of morphologically dissimilar generations: the unisexual gametophytes have a unicellular base from which 1-5 erect filaments arise. The life cycle seems to be mainly temperature-controlled. From experiments in crossed light and temperature gradients it could be concluded that spore germination and dimensions of cells and sporangia are the least modifiable and hence the most useful characters in delimiting the species from other acrochaetioid taxa. The name Acrochaetium densum was chosen rather arbitrarily. Identification of both the tetrasporophyte and the gametophyte presents considerable difficulties: a large number of described species shows resemblance to one or the other generation.