Fused isoquinoline derivatives of the general formula ##STR1## wherein: A is a divalent radical selected from the groups ##STR2## wherein R.sub.3 and R.sub.4 each independently represents hydrogen, alkyl,, alkenyl, cycloalkyl, aryl, aralkyl, aralkenyl, cycloalkyl-alkyl or, taken together with the adjacent double bond, represent a cycloalkene containing from 5 to 8 carbon atoms or an aromatic nucleus;R.sub.5 represents hydrogen, hydroxyl, alkyl, alkenyl, cycloalkyl, aryl, aralkyl, aralkenyl or cycloalkyl-alkyl;Y is .dbd.O, .dbd.S or .dbd.NH;X is --O--, --S-- or --NR.sub.6 -- wherein R.sub.6 represents hydrogen, alkyl, cycloalkyl, aryl, aralkyl or cycloalkyl-alkyl;R.sub.1 and R.sub.2 each independently represents hydrogen, alkyl, phenyl, cycloalkyl, alkoxy, alkenyloxy, methylenedioxy, cycloalkoxy, halo or di-alkylamino or, taken together with the adjacent carbon atoms of the benzene ring, form a cycloalkenyl group containing 5 or 6 carbon atoms or an aromatic nucleus fused on the benzene ring. The compounds are essentially prepared through thermal cyclization of a compound of the formula ##STR3## wherein A, X, Y, R.sub.1 and R.sub.2 have the same meanings given before, with the proviso that R.sub.1 and R.sub.2 cannot simultaneously occupy both the ortho positions, B represents hydrogen, R represents an --OR.sub.7 or --NR.sub.8 R.sub.9 radical wherein R.sub.7 is alkyl, aryl or aralkyl and R.sub.8 and R.sub.9 each independently represents hydrogen, alkyl, aryl, aralkyl, or taken together with the adjacent nitrogen atom represent a heterocyclic saturated radical of 5 or 6 atoms which may contain a further hetero-atom selected from oxygen and nitrogen, or the symbols B and R taken together represent a further bond between the adjacent carbon and nitrogen atoms.