The revised Inventory of Didemnid Ascidians from Roscoff area contains ten species instead of eleven which have been listed before (1955). Three species have been suppressed by synonymy: Trididemnum niveum (Giard), Didemnum perspicuum (Giard), Didemnum dentatum (Della Valle). A fourth species has been suppressed since it is a tropical one and does not exist on the French coasts: Didemnum candidum Savigny. Three species have been added: Polysyncraton biolbatum Lafargue which was confused with Didemnum fulgens (Giard); Didemnum coriaceum (Von Drasche) which was confused with Didemnum maculosum/i> (Milne Edwards); Diplosoma spongiforme (Giard) which was confused with Diplosoma listerianum (Milne Edwards). One species has moved to another genus: Didemnum perforatum (Giard) belongs to the genus Lissoclinum in spite of the misleading appearance of the colonies.