Field experiment was conducted to investigate the possibility of exploiting rubber (Hevea brasiliensis) for latex at a lower stem girth (40 and 45 cm) and physiological yield parameters were monitored to establish factors responsible for differences in yield due to opening girths or tapping systems tested. Three different Hevea genotypes, i.e. RRISL 211, RRIC 121 and RRIC 102 were exploited at different growth stages, i.e. girth of 40 cm (G₄₀), girth of 45 cm (G₄₅) and girth of 50 cm (G₅₀), using different tapping systems. Significant clonal differences were observed for the initial rate of latex flow (IFR), plugging index (PI), total volume of latex (vol), dry rubber content (DRC) and dry rubber yield when tapping commenced at different opening girths. The overall results of the present study clearly showed that the plugging index was relatively high in trees opened at lower girths, i.e. 40 cm (G₄₀) than in trees opened at higher girths, i.e. G₄₅ and G₅₀. Yield differences between trees of the same clone were mainly caused by differences in IFR, while differences between clones were mainly associated with differences in flow time. There is a highly significant positive correlation between PI and DRC irrespective of the tapping systems and opening girths. This study clearly showed that the DRC, PI and IFR vary according to year of tapping, tapping system and opening girth. At the immature phase (especially G₄₀ trees during the first year) were probably not mature enough to show normal latex flow relationships. Furthermore, variations in yield determining parameters have to be taken in to account in devising strategies for yield improvement in different clones.