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Fifty-one species of Crustacea are recorded from plankton samples taken from the Sokoto river system. The following new species are described; Alona. holdni, Microcyclops pseudo-anceps, Cyclopa gauthieri, Phyaocypria minicapensis. An account is given of the seaaonal occurrence of each species in the main channel of the river and in a pool which becomes isolated from the river during the dry season. Two assemblages of Crustacea are distinguished; in the dry mason Diaphanosoma excisum, Moina dubicr, two species of Thermocyclops and Tropodiaptomus laurentii form the bulk of the plankton, but in the wet mason, when the river is in flood the dominant crustaceem are members of the cledoceran genus Alona. The specific composition of the crustacean plankton of the Sokoto is compared with that of the Illinois and the Nile; in all three rivers there are approximately three times as many species of Cladocera as of cyclopoid copepods.