The morphology and phenology of Paralemanea mexicana (Kützing) Vis et Sheath was evaluated seasonally in a fifth order high-altitude stream in central Mexico. The gametophytes grew during oligotrophic and eutrophic conditions, and during particular microhabitat conditions: high current velocity (40–240 cm s−1), low to medium irradiance (5–973 μmol photons m−2 s−1), and shallow depth (1–30 cm). The abundance of gametophytes was positively correlated with low temperature, high current velocity and concentrations of soluble reactive phosphorus. Interestingly, monoecious gametophytes formed two types of branches, true branches with sympodial pattern originating from meristematic cells in variable number, and abundant false branches produced during the development of the ‘Chantransia’ stage in the surface of the gametophyte or by uniseriate filaments arising within the thallus lumen. These filaments generally produce gametophytes and suggest that they could support the germination of carpospores. The frequently whorled branches are the result of a false branching pattern and are exclusive to P. mexicana. These morphological and reproductive characteristics appear to be biomechanical adaptations to avoid detachment and increase reproductive success. Efficient reproductive strategies of P. mexicana observed in this study can be interpreted as adaptations to successfully colonize streams; however, these features may not have been common in the study region due to restricted microhabitat conditions and geographic isolation.