1. The soluble proteins of liver, blood, and ovary of the zebrafish,Brachydanio rerio, were separated by electrophoresis on cellulose acetate strips (cellogel) and polyacrylamide gels (6%). The ontogenesis of specific protein spectra was studied in females of 1, 4 and 6 months and compared with adult males. In females the development of distinct protein patterns correlated with the first vitellogenic cycle could be observed. 2. In the course of the incorporation of the yolk system (known from previous cytological investigations) 2 protein fractions identified as yolk materials appear in the ovary. 3. The first of these fractions is present already in ovary homogenates from females of 4 months. It consists of a glycoprotein with a small electronegative charge at pH 9. As this fraction is found only in the ovary, it represents endogenous yolk material. 4. In the 6 months old adult females an additional band appears in the ovary. This becomes the dominating fraction and was identified as an electronegatively charged lipoprotein. Fractions with corresponding properties are also found to a slight extent in the serum and can be extracted from the liver. That material therefore represents exogenous yolk. Probably the liver is the site of vitellogenin synthesis. 5. The endo- and the exogenous yolk substances have the same electrophoretic mobility in polyacrylamide gel. 6. The endogenous yolk is certainly, the exogenous probably, a female specific protein.