ABSTRACT Experiments were conducted with water samples from two perialpine lakes with differing eutrophication status in order to examine the effects of inorganic-nutrient amendments (nitrogen as NO ₃ ⁻ or NH ₄ ⁺ and phosphorus as PO ₄ ³⁻ ) on the dynamics, structure, and composition of Planctomycetes and to test the hypothesis that the community structure of Planctomycetes members and that of the other bacteria (without Planctomycetes , here referred to as bacteria-wP, the most represented groups within the community) would be similarly impacted by nutrient additions. Initial samples were characterized by high total nitrogen-to-total phosphorus ratios (range, 39 to 55), suggesting P rather than N was the limiting nutrient for microbial communities. Consistent with this, P additions stimulated phytoplankton growth and affected the community structure of bacteria-wP but, surprisingly, not that of Planctomycetes . N additions did not significantly affect the community structures of bacteria-wP and Planctomycetes or the Planctomycetes phylotype composition. The estimated generation time of Planctomycetes was 123 h. These findings could suggest that the generally well-accepted statement that bacteria (as a whole) are superior to phytoplankton in the ability to obtain phosphorus under P limitation might actually not hold for Planctomycetes . Planctomycetes might be poor competitors for P that do not respond quickly to the nutrient supply, which may help explain why their abundance is low in aquatic systems. The alternative view that Planctomycetes could be strong competitors for P (storing it) is also discussed. Our findings highlight the need for further studies examining Planctomycetes -phosphorus relationships in aquatic ecosystems.