Females of Bombyx mori heterozygous for the Rg deletion and the sm gene tend to lay either normal or small eggs, a single female rarely produces both types of eggs. This is explained as due to S-type position effect manifested through the right break of Rg deletion situated just left of the sm locus. Only one case of the exception was observed after the examination of more than two thousand females. One female of the genotype Rg/sm produced irregular shaped eggs of normal size and small. Protein pattern of larger eggs was similar to that of normal eggs, whereas protein constitution of smaller eggs rather resembled to sm eggs. It is assumed that this exceptional female may involve, in her ovaries, groups of cells functionally normal and sm which were differentiated through V-type position effect.