The expression of the fs(1)K10 gene is required in early oogenesis for the establishment of the dorsal-ventral polarity of the oocyte, and later in the embryo. P-element-mediated transformation shows that the K10 function is located within a fragment of DNA of 5 kb, which encodes four RNA species. A major transcript of 3.1 kb is likely to be responsible for the K10 function. It is abundant in ovaries and in early developing embryos. Thus its expression profile corresponds closely to that which could be anticipated from the biological characteristics of the mutation. In situ hybridization on ovary sections shows that the gene is not only specifically transcribed in the germ line (which is consistent with the germ-line dependence of the mutation), but that its expression is also cell-specific since it is apparently restricted to the oocyte.