The nature and magnitude of genetic variability, heritability and heterosis were assessed in 23 F1 hybrid clones, derived from the cross between RRII 105 and RRIC 100 of Hevea brasiliensis, at the premature phase of 4 1/2 years after planting. The hybrid clones displayed substantial differences and heterotic responses of various characters. Genetic variability was highest for total volume of latex (GCV 42.7;) and lowest for bark thickness (GCV 4.24;). Mean annual yield exhibited a heritability of 68;with a genetic advance of 59 per cent. Yield following ethrel stimulation, initial flow rate per unit length of tapping cut and plugging index recorded high heritability coupled with high genetic advance. The high heritability and genetic advance suggest preponderance of additive gene action. Among the 23 hybrids, 15 outyielded their superior parent (RRII 105),of which six displayed significant heterosis over RRII 105.