The water mite Unionicola penicillatus (Wen, Gao & Zhu) (Acari: Unionicolidae) was collected in freshwater bivalve Cristaria plicata (Unionidae). The population variation of U. penicillatus was investigated in 504 bivalves during the period from August 2013 to July 2014 in Poyang Lake, East China. The results indicated that the overall prevalence and abundance of U. penicillatus was 9.52% and 1.58±10.38. The highest prevalence was 58.70% in September. The highest abundance levels were observed when the length of bivalves was in 220– 300 mm. The number of U. penicillatus in a host was not correlated with the sex of bivalves. The outer and inner gills of bivalves were the preferred sites of U. penicillatus.