In decades, Pseudanabaena species have been frequently reported as cyanobacterial bloom components. However, these species are always overlooked because they cannot be observed easily owing to their small sizes. This study aimed to characterize Pseudanabaena species and detect them in field waters by developing an effective DNA marker. A region with approximately 600 bp in the rbcLX sequences was specific to Pseudanabaena species alone compared with other cyanobacterial genera. Therefore, new primers specific for Pseudanabaena were generated. By using this molecular tool, seasonal and spatial dynamics of Pseudanabaena populations of six sites in Lushui Reservoir, China were measured from July 2010 to August 2011. Pseudanabaena species existed in the reservoir throughout the year, and cell abundance ranged between 10(4) and 10(6) equivalent cells/L, with the peak at 8.07 x 10(6) equivalent cells/L The seasonal dynamics pattern of Pseudanabaena was similar to that of Microcystis but showed a slight lag at the transitional points of water temperature in the reservoir. (C) 2015 Elsevier Ltd. All rights reserved.