The invention concerns the use in a cosmetic composition of at least a poly-aminoacid of general formula (I) in which X is --O--, --S-- or --NR.sub.3; R.sub.1 represents in particular a hydrogen atom or a C.sub.1-C.sub.40 alkyl radical; R.sub.2 represents in particular a hydrogen atom or an optionally substituted alkyl radical; R.sub.3 represents a hydrogen atom or an alkyl radical; R.sub.4 represents a hydrogen atom or a radical such as NH.sub.2, OH, SH, --CHOHCH.sub.3, --CONH.sub.2; n is a number greater than 1 such that the poly-aminoacid derivative molecular weight ranges between 200 and 200000. The invention also concerns the use of said derivatives for keratinous fibre strengthening and care ##STR00001##