Eucalyptus is the most widely planted species in Madagascar. Eucalyptus robusta is the most common species (225 000 ha) because it is well adapted and has multipurpose wood uses (energy, pole, and timber). Although this species has been naturalized for more than a century, little is known about the introduced genetic diversity. Previous research leads us to assume that it has a narrow genetic base. A study of genetic variability and genotype by environment interaction for E. robusta growth was therefore conducted. This study concerns two provenance trials (24 provenances from Australia, 1 from Madagascar) and three progeny trials established in two bioclimatic regions, regularly monitored for >10 years. In both sites, results showed strong provenance effects and genotype by environment interactions for growth. However, the Malagasy provenance was the poorer performer for growth, compared to the Australian. This could be attributed to its origin, mainly to inbreeding depressions due to the narrow genetic base introduced. Due to climate change, this low diversity, which will be confi rmed by ongoing molecular genetic studies, may represent biological and economical risks. These will help forest managers to capitalize on the genetic resources existing in Madagascar for a better future. (Texte intégral)