Taxitheliella is a monotypic genus endemic to Sarawak (Borneo), and the species, T. richardsii Dixon, is only known from the type collection made by P.W. Richards in August 1932. The lack of recent collec-tions, in spite of several recent searches made in Sarawak, and the high rate of habitat loss in Borneo have placed this species as critically endangered by the IAB Bryophyte Specialist Group (Tan et al., 2000). Since its inception, this poorly known genus has been placed in different families, such as Semato-phyllaceae (Dixon, 1935), Hypnaceae (Tan et al., 2000; Interactive Malesian Moss Database, 2009), Pylai-siadelphaceae (Goffinet & Buck, 2004; Goffinet et al., 2009), and is listed as Fabroniaceae in TROPICOS (www.tropicos.org). Dixon (1935) had considered its only species (T. richardsii) closely related to the genus Taxithelium, and therefore a member of the family Sematophyllaceae sensu lato. His opinion was basically influenced by the presence of multiple leaf cell outgrowths, a distinctive character in Taxithelium. However, in the same publication he recognized also the similarity of the sporophyte characters with those seen in Fabroniaceae. The presence, kind, and number of these outgrowths (usually named papillae in Sematophyllaceae) per leaf cell have long been considered a good taxonomic character for moss classification. In this study, we investigated the morphology of leaf cell outgrowths in Taxitheliella and discuss the familial placement of this remarkable North Borneo endemic moss. To examine the leaf cell outgrowths in T. richardsii, samples were prepared according to the protocol suggested by Bozzola and Russell (1998) and followed by Câ mara & Kellogg (2010). Specimens were digitally photographed with a Hitachi S-2600H SEM at 20 kV. All measurements were taken from the SEM pictures. A more detailed morphological study of T. richard-sii was also performed. Capsules and parts of the gametophyte were dissected and slides were prepared with Hoyer's solution (Anderson, 1954), and char-acters were investigated under a light microscope. There are multiple outgrowths in the cell wall in 2– 3, rarely 4, rows over the lumina. The outgrowths are about 1.7–2.2 mm in diameter and ca 2.0 mm in height. These outgrowths are only present on the abaxial surface of the leaf. It was possible to distinguish three distinct forms (Figures 1 and 2) of leaf cell outgrowths: (1) single outgrowth; (2) in groups of two; and (3) fused outgrowth as wide as the cell width. At both ends of the leaf cells, the outgrowths are always single; but at the widest point of some leaf cells, they are sometimes paired (Figure 1). Also, it was observed that at the widest point of some leaf cells, a single outgrowth can be seen to be as wide as the cell width, but maintaining the same height (Figure 2). When the preparation was done without the use of critical point drying, the outgrowths collapsed together with the cell wall. Additionally, we observed a number of morpholo-gical characters not mentioned in the protologue. We therefore provide below a more complete species description.holotype: BM!, isotype E). Plants small, forming mats. Stems creeping, irregu-larly branched to subpinnate; central strand absent; pseudoparaphyllia absent; branches complanate or terete. Stem and branch leaves usually similar, straight, broadly oblong–ovate, 0.11–0.2260.45–0.50 mm; mar-gins entire; apex obtuse to acute; costa absent; laminal cells linear, 5.4–9.3637.9–60.3 mm, seriately papillose over lumina, thin-walled; differentiated alar cells few, never inflated or colored. Asexual propagula absent.