Sixteen families derived from a clonal seed orchard (CSO) and 10 "provenances" (Prov) of teak (Tectona grandis) were tested in two different sites to be compared with respect to their growth performances. Both sites were located in Sabah, East Malaysia, under 2,500 mm of annual rainfall and no distinct dry season. The land in Taliwas was flat but prone to waterlogging. In Luasong, the soil was more hilly, acidic, and less fertile, though deeper. Nearly 9 years after planting, the two classes of genetic entries showed significant differences for height (P=0.0002) and diameter at breast height (DBH) and volume (P<0.0001) for the two sites combined. The superiority of the CSO families compared with the Prov class was more obvious in Luasong with averages of 18.0 vs 15.2 m (+18.6 %) for height, 18.1 vs 15.1 cm (+20.2 %) for DBH, and 0.179 vs 0.107 m3 for individual tree volume (+67.9 %), as against 17.4 vs 15.7 m (+11.2 %), 19.1 vs 16.8 cm (+13.5 %), and 0.176 vs 0.126m3 (+40.3%), respectively, in Taliwas. The CSO families were also more prone to site interaction for height (P=0.004) and, to a lesser extent, for volume (P=0.017) than the "Prov" (P=0.030 and P=0.057, respectively). Narrow-sense heritabilities estimated for the 16 CSO families across the two sites were lower for DBH (0.17) and volume (0.23) than for height (0.38). Type B genetic correlations suggested also higher site × families interactions for height (rB=0.28).