A method of analytic separation of soluble proteins in aqueous extracts of Plasmodium berghei is described. Extracts are prepared by sonication of suspensions of whole washed parasites obtained by saponin lysis of infected rat red cells. Separation of the extracts into reproducible electrophoretic patterns is carried out by zone electrophoresis on polyacrylamide gel. Extracts prepared in this manner contain a heterogeneous mixture of soluble protein constituents of both host and parasite origin. A minimum of 15 parasite proteins and an additional minimum of six proteins of host origin have been identified. This paper presents an analytic separation of water soluble proteins of Plasmodium berghei by the technique of disk electrophoresis. Electrophoresis is carried out in an acrylamide gel matrix of defined pore size permitting separation of components simultaneously by differences in charge and in size and shape. Application of this method to extracts of P. berghei has provided confirmation and extension of reported findings by agar immunoelectrophoretic methods that such extracts are a complex mixture of proteins (Zuckerman, 1964; Sherman, 1964).