We investigated the occurrence of unionid bivalves during the non-irrigation season of 2013 at 132 locations in two agricultural channels in Kawajima Town, Saitama Prefecture, where a small number of the alien species Lanceolaria grayana(Lea, 1834)was first found in 2005. We found a total of 255 individuals at 56 locations widely distributed throughout the channels. Generalized linear models revealed that the density of L. grayana depends mostly on water depth and to a lesser extent on substrate type, but not on substrate compaction, current velocity, vegetation cover ratio, electrical conductivity, or dissolved oxygen. We found three native unionid species(Pronodularia japanensis, Unio douglasiae nipponensis, and Anodonta sp.), densities of which were much lower than that of L. grayana. Further studies are needed to address to what extent L. grayana has spread to surrounding areas and its impact on native unionid species.