The concept of core collections as developed by Brown (1989a) would be very useful in optimizing conservation strategies of natural population of outbreeding grasses. The aim of a core is to represent, in a subsample of manageable size, as much as possible of the genetic variation from a large collection. In the case of natural populations of ryegrass, different methods of stratified sampling with one level of classification have been compared: 1. Random sampling. 2. Clustering based on agronomic traits. 3. Clustering based on the administrative region of origin. 4. Clustering based on agronomic traits with geographic contiguity constraint.