Spatial distribution and seasonal variation in mean intensity and prevalence of monogeneans on the gills of grass carp (Ctenopharyngodon idellus) were investigated from September 2012 to December 2014 in a fish pond in Wuhan, China. During this period, 2 species of Dactylogyrus were found, i.e., Dactylogyrus lamellatus and Dactylogyrus ctenopharyngodonis. Dactylogyrus lamellatus was present during the entire duration of the investigation, whereas D. ctenopharyngodonis was only detected after January 2014, when another batch of grass carp fingerlings was added to the pond. Prevalence of the 2 Dactylogyrus spp. was relatively high throughout the year, but significant seasonal changes were detected in the mean intensity of the 2 species (P < 0.05). Mean intensity of D. lamellatus peaked in late winter and spring, and then dropped to the lowest point in summer. Dactylogyrus ctenopharyngodonis exhibited a high mean intensity in summer and autumn, but low in winter and spring. Spatial distribution of the 2 species was similar: the highest mean intensity was found on medial and distal parts of the second gill arch. Moreover, no evident change was detected in niche preference of D. lamellatus after the infection of D. ctenopharyngodonis occurred. There was no significant positive correlation in abundance between the 2 species (P > 0.05). In addition, there were no significant differences in the percentage of mean intensity of the 2 species on each gill arch among low-, medium-, and high-intensity groups (P > 0.05). These results suggest unsaturated gill niches and the absence of interspecific competition between the 2 Dactylogyrus species.