In this paper, the authors analyse the chromosomes of a field population ofBilobella aurantiacafrom Sainte Baume (Southern France). They display a low polymorphism and high content of heterochromatin, in contrast to populations of central Spain. The observation of ectopic pairing suggests the presence of two types of heterochromatin: telomeric heterochromatin is frequently involved in ectopic pairing, while centromeric heterochromatin never is. The amount of heterochromatin and the extent of polymorphism seem to vary in opposite directions from one population to an other according to general climatic conditions: the existence of two different adaptative strategies is proposed on this basis.