A genotypic scale, which is additive, has been worked out for variation of scutellar bristle number in D. melamoqaster, sc+ is shown to be partially dominant to scs�, the first substitution of SCSC for sc+ having about six times more effect on genotypic value than the second. Canalization at four bristles is responsible for complete dominance of sc+ in stocks whose mean genotypic value gives a mean phenotype of four bristles in sc+ flies. A function converting the phenotypic to the genotypic scale is given which could be used in heritability analysis.