Cereals and legumes that originated from the semi-arid tropics (SAT) are mainly grown in marginal soils with limited water and nutrient resources. Initial root system development is crucial for crop establishment in order to acquire these minimal resources. A comparative study of the root system morphology and of some physiological parameters at initial growth stages was carried out for 3 legumes: pigeonpea, chickpea and groundnut, and 3 cereals: sorghum, pearl millet and maize, the component crops of the semi-arid tropics. Considerable differences were observed for all the root morphological traits among the species. Among the legumes, chickpea produced few thick and lengthy laterals, while pigeonpea produced a high frequency of thinner laterals. However groundnut had a larger root system than the other 2 legumes. Among the cereals, maize formed a large root system followed by sorghum with a high frequency of laterals, while millet displayed a smaller root system with thin roots. The root respiration rate was significantly correlated with the N uptake activity. The legumes showed a higher efficiency of N uptake in terms of respiratory requirement. The transpiration rate showed a significant correlation with total N concentration in shoot, indicating that transpiration may be partly related to nitrogen flow to and accumulation in shoot in case of nitrate nitrogen as a sole nitrogen source. Morphological and physiological characters of the root system using rather simple indices were found to be better criteria for describing functional differences among crop species.