The phthalidyl-isoquinoline derivatives have the formula: ##STR1## in which R.sub.1, R.sub.2, R.sub.4, R.sub.5 and R.sub.6 each denotes a lower alkoxy group, R.sub.3 denotes a hydrogen atom or a lower alkoxy group and R.sub.7 denotes a lower alkyl group and also include their diastereoisomers and their addition salts with pharmaceutically acceptable acids. These compounds have antiallergic activities and may be used to treat allergic symptoms in human beings and other animals.