A naphtholactam dyestuff of the general formula (I) ##STR1## in which R represents alkyl, alkenyl, cycloalkyl, aralkyl, aryl or, preferably, hydrogen, M represents hydrogen or a cation, X represents .dbd.O, .dbd.NH or .dbd.NR', Y represents a free or modified carboxyl group, Z represents a non-ionic substituent, n represents 0, 1 or 2, B represents a group which completes ring A to give a heterocyclic structure, R' represents alkyl, alkenyl, cycloalkyl, aralkyl, aryl, or, preferably, hydrogen and E and F can carry further non-ionic substituents.The new dyestuffs give brilliant yellow and red shades on polyester fibres.