Acridanone derivatives of the formula ##STR1## wherein the dotted line is an optional bond, R.sup.1 is hydrogen, halogen or nitro, R.sup.2 is hydrogen or lower alkyl, one of the substituents R.sup.3 and R.sup.4 is hydrogen or lower alkyl and the other together with R is an additional bond and R.sup.5 is hydrogen, lower alkyl, lower alkenyl, lower alkynyl or lower alkyl substituted by halogen or lower alkoxy,and pharmaceutically acceptable acid addition salts thereof, processes for their preparation and pharmaceutical compositions based thereon, are described. The compounds of formula I are useful as schistosomicidal agents.