The susceptible nature of rubber wood to sapstain fungus limits its wider utilisation. A survey has shown that in Kerala, due to favourable environmental conditions, sapstain caused by B. theobromae is a serious problem and rubber wood is highly susceptible to fungal stain. Laboratory studies revealed that there was a weight loss of 8;in rubber wood block inoculated with B. theobromae during the first month which increased to 12;by the end of the fourth month. The growth of B. theobromae on rubber wood of different moisture contents was greatly affected at different temperature and relative humidity ranges. Cent percent growth of B. theobromae occurred at 100;RH and two temperatures viz., 20 degree and 30 degree C. An increase in temperature (eg. 40 degree C) even with 100;RH reduced considerably the growth of B. theobromae. No growth of the fungus was recorded at any of the temperatures (20, 30 and 40 degree C) at RH 60 and 70;. Higher moisture content of rubber wood favoured good growth of B. theobromae in comparison with low moisture content especially when the RH was 100;.