Résumé : Growth and heavy metal (Zn and Cd) hyperaccumulation were investigated in metallicolous and nonmetallicolous Mediterranean populations of Thlaspi caerulescens (Brassicaceae), and in offspring from controlled crosses between these populations. Seeds for the growth and crossing experiments were collected from a number of sites varying in heavy metal contamination. Tissue Zn and Cd content was determined by atomic absorption spectrophotometry. Offspring from crosses between nonmetallicolous populations had the highest Zn concentration (c. 30 000 μg g -1), compared with 20 000 μg g -1 for the nonmetallicolous parents. The metallicolous parents and the other crosses had only 10 000 μg g -1. Offspring from crosses including a nonmetallicolous parent still had a significantly higher Zn uptake than the metallicolous parents. A trend towards a higher Cd uptake was observed in offspring from crosses with a metallicolous parent. We suggest that the most probable hypothesis is that the differences in Zn hyperaccumulation between crosses could be explained by a monogenic system with two alleles. The dominant allele would restrict Zn hyperaccumulation at 10 000 μg g -1 whereas the recessive allele would be responsible for a two to three-fold increase in Zn hyperaccumulation. Alternatively, the existence of modifier genes could explain the differences between offspring from crosses between nonmetallicolous populations and their respective field parents. The results suggest that plant breeding applied to this species could help to improve Zn phytoextraction. © New Phytologist (2003).