Experiments were carried out to analyze the root and water dynamics simultaneously using a minirhizotron on an Alfisol soil in the semi-arid tropics in 1993. Sorghun (CSH 5), pearl millet (ICMV 221), pigeonpea (ICP 1-6), groundnut (ICGS 11) and cowpea (EC 82-7) were used to describe the relationship between the root length density (RLD) and water uptake by roots per day (WU). During the periods from 37 to 46, from 51 to 59 and from 72 to 77 days after sowing, the average values of RLD of sorghum and pearl millet at the soil depth of 15-60 cm were consistently higher than those of pigeonpea, groundnut and cowpea, whereas the average values of WU of the cereals were not always higher than those of the grain legumes. Thus, there was no significant relationship between RLD and WU for the 5 crops due to the higher values of the specific root water uptake (SRWU) of grain legumes than those of cereals in each period. In this study, it was demonstrated that WU and SRWU as well as RLD for the 5 crops could be estimated by using the minirhizotron since dynamic values of the root length of the 5 crops and soil moisture content (SMC) in each soil layer could be quantified from datasets by frequent observations and with limited sampling errors. Therefore the minirhizotron was found to be a suitable tool for simultaneous monitoring of the root and water dynamics in soil layers except for the soil surface.