Stringent environmental regulations in California have necessitated the design of cropping systems that use water more efficiently, conserve the soil, reduce dust emissions, and reduce herbicide use. To address these issues, conservation tillage (CT) and subsurface drip irrigation (SDI) are methods currently being investigated in some vegetable crops. Because CT may limit weed emergence to the top few inches of soil and SDI may keep the soil surface too dry for weed emergence, these methods may have implications for weed suppressive cropping systems in arid regions. However, weed population dynamics as an interaction of SDI and CT has not been adequately explored. A study is being conducted at Five Points, CA to assess the effect of SDI and CT on weed population dynamics in processing tomato (Lycopersicon esculentum Mill.). The experimental design is a split-split plot with four replications. Tillage system (CT vs standard) is the main plot, irrigation system (SDI vs furrow) is the sub-plot, and herbicide (herbicide vs no herbicide) is the sub-sub-plot. First year results showed that tillage had no effect (P>0.05) on weed densities. Weed densities were 84 to 92% lower (P<0.01) in the SDI compared to the furrow irrigated plots. The SDI plots had more than 95% of the weeds concentrated in or near the crop row whereas, the furrow irrigated plots had weeds in the crop row, on and between the beds. Weed biomass at the end of the season was about 70% lower (P <0.0001) in the SDI than the furrow irrigated plots. Total crop yield was not affected (P>0.05) by tillage or irrigation system. Although the fruit quality was not affected (P>0.05) by the irrigation system, there were more (P<0.05) red fruits and less (P<0.05) rotten fruits in the furrow compared to the SDI plots. The experiment is being continued in 2005.