Abstract: The invention relates to a novel lipid extract of the algae Skeletonema, especially the algae Skeletonema costatum. In particular, this extract is a total lipid extract of said algae. It can be obtained by extracting the algae Skeletonema in an organic solvent which has a polarity index p′ of less than about 5.4, preferably of between 2 and 4.5 and particularly preferably of between 4.2 and 4.4, and which is acceptable in the cosmetic or pharmaceutical industry. This extract can be used as an active principle for the manufacture of a cosmetic or pharmaceutical composition particularly for producing a slimming, anti-cellulite, skin anti-ageing or sensitive skin treatment.