An isothiazolylazo dye of the formula I ##STR1## wherein R is an alkyl group or unsubstituted aryl or substituted aryl group selected from the group consisting of ##STR2## and K is a radical of the formula ##STR3## where B is C.sub.1 -C.sub.4 alkyl or a substituted amino radical selected from the group consisting of ##STR4## R.sup.1 is hydrogen, methyl, methoxy or ethoxy, R.sup.2 and R.sup.3 independently of one another are each unsubstituted C.sub.1 -C.sub.4 alkyl or substituted alkyl groups selected from the group of C.sub.1 -C.sub.4 alkyls which are substituted by chlorine, bromine, cyano, hydroxyl, C.sub.1 -C.sub.4 -alkoxy, benzyloxy, phenoxy, phenyl, C.sub.1 -C.sub.4 -alkylcarbonyloxy which is unsubstituted or substituted by C.sub.1 -C.sub.4 -alkoxy, phenoxy or phenyl, phenycarbonyloxy which is unsubstituted or substituted by chlorine, bromine, methoxy, ethoxy, methyl or ethyl, oxycarbonyl, C.sub.1 -C.sub.4 -alkoxycarbonyl, C.sub.1 -C.sub.4 -alkoxycarbonyloxy, carbamyl, phenalkyloxycarbonyloxy, phenoxyalkyloxycarbonyloxy, C.sub.1 -C.sub.4 -alkoxyalkoxycarbonyloxy, phenylaminocarbonyloxy which is unsubstituted or substituted by chlorine, methoxy or methyl, C.sub.1 -C.sub.4 -alkoxyalkoxycarbonyl, phenylalkyloxycarbonyl or C.sub.1 -C.sub.4 -alkylaminocarbonyloxy which is unsubstituted or substituted by C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.4 -alkoxy, chlorine or bromine; or R.sup.3 is hydrogen, R.sup.4 is a radical of the formula --A--OCO--Y--R, Z is hydrogen or methyl, X is hydrogen, methyl, methoxy, chlorine or an acylamino radical selected from the group consisting of A is C.sub.2 - or C.sub.3 -alkylene and Y is --O-- or --NH--. The dyes are very useful for dyeing cellulose esters and synthetic polyesters; the dyeings obtained are from yellow to blue, and very fast.