To investigate factors and mechanisms regulating toxin-producing populations of Planktothrix, we conducted a field study (2001-04) in the mesotrophic Lake Steinsfjorden, South-eastern Norway. The occurring species, Planktothrix rubescens and P. agardhii, had similar depth distributions and seasonal dynamics, both forming metalimnetic blooms in 10-14 m depth. By comparing the resource availability and temperature in Lake Steinsfjorden with demands determined in laboratory studies, temperature and light were identified as the most important factors controlling growth and depth distribution of Planktothrix spp. In addition, macronutrients, especially nitrogen, may have limited growth in periods. A lowering of nutrient supplies over time could in addition to the prevailing suboptimal temperature and light conditions prevent the population of Planktothrix spp. from forming blooms. On two occasions, a major decrease in Planktothrix spp. abundance in the open water could be linked to a transport towards the banks of Lake Steinsfjorden with subsequent decomposition in the littoral zone. Our results show that the depth distribution and seasonal dynamics of Planktothrix spp. in Lake Steinsfjorden is controlled by environmental factors in a similar way as in other Nordic, Central European and North American lakes.