The indusia of 29 well documented Dutch plants of the Dryopteris affinis aggregate, with known ploidy, have been studied before, during, and after ripening. It is concluded that, in addition to leaf morphometric features, the indusium features mentioned in British, German and French Floras to distinguish between the two currently recognized species – or subspecies – of the aggregate occurring in the Netherlands, i.e. Dryopteris (affinis subsp.) affinis and D. (affinis subsp.) borreri, are also useful discriminating characters in the Netherlands. These characters can be used for identification in the field as well.