Leptocybe invasa Fisher & La Salle (Hymenoptera: Eulophidae), also known as Blue Gum Chalcid (BGC) is native to Australia. The pest was first recorded in the Mediterranean region in 2000 causing severe injury to young foliage of Eucalyptus camaldulensis by inducing galls mainly on growing shoots. The pest was first reported in Tanzania in 2005 and has recently become a problem by infesting a range of commercially grown Eucalyptus species. The purpose of this study was to determine infestation density and assess the effects of L. invasa on growth, biomass production and wood basic density of some Eucalyptus species in Tanzania. Results showed that, trees in the Coastal were more infested, followed by Plateaux while trees in the Southern highlands agro-ecological zone were least infested. Eucalyptus tereticornis was more infested followed by E. camaldulensis while E. saligna was the least infested. Eucalyptus citriodora and E. grandis were not infested. The mean Dbh of infested trees were reduced by 7.8%, 2.1% and 13.6% and mean heights were reduced by 6.6%, 9.5% and 3.8% compared to uninfested ones for E. camaldulensis, E. tereticornis and E. saligna respectively. The mean basal area of infested trees were reduced by 16.4%, 17.1% and 24.5% and mean volume were reduced by 17.8%, 16.1% and 23.1% for E. camaldulensis, E. tereticornis and E. saligna respectively. The mean basic densities of infested eucalypt trees were higher than uninfested by 2.7%, 5.3% and 7.3% for E. tereticornis, E. camaldulensis and E. saligna respectively. Twenty two weeks after L. invasa infestation, the height and root collar diameter of infested seedlings were reduced by 39.6% and 11.3% for E. grandis and by 38.2% and 7.7% for E. saligna respectively. Dry weight biomass reduction in infested seedlings was significantly higher on stem and leaves than roots and branches of both E. grandis and E. saligna. It is recommended that, similar study on the impact of L. invasa on growth and biomass production should look into other commercially grown Eucalyptus species in Tanzania. Management efforts should iii focus on controlling the spread of the pest using biological control, cultural techniques and planting resistant genotypes.Tanzanian Commission for Science and Technology (COSTECH)