The life cycle of the red alga, Furcellaria lumbricalis, is poorly known in the northern part of the brackish water Baltic Sea, even though it is one of the most common red algal species in the region. To study its life cycle and reproduction, monthly sampling (n=30/sampling site) was carried out for one year on the coast of Finland at four locations spanning 500 km along a salinity gradient (3.6–5.4 psu). In three of the populations studied, only tetrasporangia and spermatangia were produced. In the fourth population, at the lowest (3.6 psu) salinity, all algae examined were vegetative. Winter months were the main season for tetrasporangial production, when sori with masses of sporangia were present. Smaller sori with only a few tetrasporangia were observed during spring at higher salinities, and some sporangia were detected in the population at 5 psu salinity, even during the summer months. The timing and intensity of reproduction was regulated by a combination of factors including seawater salinity, along with seawater transparency, temperature and photoperiod. In conclusion, we hypothesise that three of the populations, living at salinities between 4.9–5.4 psu reproduce by thallus fragmentation and possibly by asexual tetraspore-to-tetraspore cycling without meiosis, and the fourth population, at a salinity of 3.6 psu, survives by thallus fragmentation and reattachment.