An azo dye comprising a heterocyclic diazo component, of the general formula I where D is a radical of the formula Y is nitro, chlorine or bromine, Z is hydrogen, chlorine, bromine, cyano or nitro, R1 is C1-C4-alkyl which is unsubstituted or substituted by hydroxyl, C1-C4-alkoxy, cyano, C1-C4-alkanoyloxy, C1-C4-alkoxycarbonyl, C1-C4-alkanoylamino, C1-C4-alkylaminocarbonyloxy, C1-C4-alkoxycarbonyloxy, chlorine, phenyl and/or phenoxy, or is cyclohexyl, propargyl or C3-C5-alkenyl, or is a radical of the formula (A-O)n-R2, R2 is C1-C4-alkyl, phenyl or, if n>1, may also be hydrogen, R3 is C1-C8-alkyl, which may be substituted by phenyl or phenoxy, R4 is hydrogen, methyl, ethyl, methoxy, ethoxy or phenoxy, X is oxygen or a direct bond, A is -CH2CH2-, -CH2CH2CH2-, and n is 1, 2 or 3. The dyes of the formula I are very suitable for dyeing synthetic textile materials, especially polyesters.