The aim of this work was to discuss the possibility to use rhizotrons to study the primary root ramification (number of organs) of corn plants (variety Dea). Samples, ie needle boards, have been taken 2 following years on 6 different fields chosen so that the whole genetic variability can be expressed. Measurements have also been made on rhizotrons of the following size: depth 1 m, length 20 cm, width 1 m. The substrate is the soil of one of the fields. In 1985, one-third of the boxes were fitted out with a system (described in fig 2) for selecting the roots in contact with the transparent wall. Table / gives the number of evaluation performed for each method versus year and depth. The number of secondary roots per cm of primary roots versus internode and depth is given in figures 3 and 4 for the differents methods. The variability inherent in the ramification process is important and can attain 30%. The coefficients of variation are greater for the results obtained with rhizotrons. They include the consequences of the specific perturbations due to the method. The observations made through the acrylic wall are representative of the ramification kinetics inside the box. Thus, figure 5 allows comparison of the different counts performed in the rhizotrons on the basis of the slope variation between 2 consecutive internodes. This figure shows that the acrylic wall did not disturb the secondary ramification, and that the results of the observations performed through the transparent face are proportional to those of the total number of ramifications on the observed primary roots. But these data can be extended to the field conditions for the first internodes (2E to E5) only. This is certainly caused by the soil temperature differences between the root boxes and the field which is especially important at the end of the vegetative growing season (fig 6) when the laterals on internode 6 and 7 are initiated.