Although boron (B) deficiency limits the productivity of eucalypts in plantations in many parts of the world, the concentrations of foliar B used in the diagnosis of B deficiency vary greatly among studies. There has been a lack of reliable diagnosis standards for B deficiency in Eucalyptus species. Therefore, the present study investigated the relationship between internal and external B concentrations and growth of Eucalyptus globulus, the main commercial temperate eucalypt species. Seedlings were grown in a B-buffered solution culture (Amberlite IRA 743) from 0.03 to 8.35 mgrM B. Boron deficiency symptoms appeared at day 5 in the nutrient solution containing less than 0.27 mgrM B. The external critical B concentrations, estimated for the growth of shoots and roots, were 1.08 and 0.99 mgrM B, respectively. The internal critical B concentration range in the youngest fully expanded leaf (YFEL) for shoot growth was 12–16 mg B kg–1 dry weight. The internal critical B concentrations estimated in the present study have been successfully used in the diagnosis of B deficiency in E. globulus trees up to three years of age in south-east Asia.