A field experiment was initiated in 2006-07 at Kanke centre of AICRP on cropping systems. The present study was conducted to observe the effect of intensification of rice-fallow cropping system on weed dynamics in transplanted rice var. Lalat in 2008 and 2009 i.e. 4th and 5th year. Results showed that intensification of rice - fallow cropping system at 300% cropping intensity considerably reduced the weed population and weed dry matter production as compared to 200% (rice-wheat) and 100% (rice-fallow) cropping systems. Among the cropping systems tested, rice - potato + wheat (1:1) - green gram recorded significantly lowest weed population and weed dry matter (at par with rice-potato-green gram). Intensification of rice-fallow was found to produce higher production and productivity of rice. Nevertheless, the yield differences were significant only in 2009 during which rice - potato - green gram recorded maximum rice grain yield being at par with the grain yield in rice - potato + wheat (1:1) - green gram. Effect of cropping system on harvest index of rice crop was also significantly higher at 300% intensification in rice - potato + wheat (1:1) - green gram and rice - potato - green gram over others