Investigations on population dynamics against thrips on castor was carried out during the year 2002-03 and 2003-04 at Main Oilseeds Research Station, Junagadh Agricultural University, Junagadh. Peak activity was observed in 44th standard week (first week of November) and 42nd standard week (third week of October) by registering 196 and 193 thrips per flag leaf during 2002-03 and 2003-04, respectively. Maximum temperature, sunshine hours and evaporation exhibited significant positive correlation whereas, morning/afternoon humidity showed significant negative association. Path analysis showed that maximum / minimum temperature exhibited highly significant positive direct effect. Stepwise regression analysis further revealed that an increase in 1°C of maximum temperature increased the thrips population by 18.44 and 16.51 per flag leaf during 2002-03 and 2003-04, respectively, whereas a unit increase in minimum temperature and morning relative humidity reduced the thrips population to the tune of 7.19 and 2.22 per flag leaf during 2002-03 and 2003-04, respectively. The requirements of heat units for thrips build up and to attain the peaks were almost similar in both the years. Relatively higher heat units in the month of October with the end of rainy season indicated the build up of thrips population.