Abstract: In April 2002, 33 species of planktonic crustacea in 26 genera (11 Copepoda and 22 Cladocera) were sampled from 17 sites in the middle and lower reaches of the Hanjiang River, the largest tributary in the middle reaches of the Yangtze River. ShannonWiener, Simpson, and Pielou indexes calculated from species composition and abundance were used for evaluating the community diversity characteristics in different reaches. Spatial variations of the communities in different reaches were compared by method of SelfOrganizing Map (SOM) artificial neuronal networks. The results were summarized as followings: (a) the density of planktonic crustacea in middle reach was higher than that in the lower reach whereas the species abundance and community diversity index in the lower reach were higher than that in the middle reach; (b) the planktonic crustacea communities presented high density but lower diversity in the upper part of the middle reach, lower density but high diversity in the lower part of the middle reach and estuary, and lower density and diversity in the lower reach except the estuary.

Key words: planktonic crustacea, spatial variation in community, SOM, the middle and lower Hanjiang River