To help with the diagnostic of the Malian Cotton small-scale farmers fields, an inexpensive means is needed to measure the Leaf Area Index. A simple method is proposed and tested. It rests on the interpretation of digital photographs, taken with any commercially available camera. The photographs are taken from above the rows. White blankets on the soil are used to facilitate the distinction between the leaves and the ground, as well as to define and standardize the frame of the image. The framed photographs are then contrasted and transformed into bitmap images. The proportion of black points, Index of Black (IB), calculated with a simple program written on a shareware is used as an indicator of LAI. This indicator was tested on 3 trials in 2002, sown at different densities. The LAI was measured with an independent method on the same plots. The results of the comparison between the two methods show that the IB is a good LAI indicator as long as LAI is less than 2. Above this value, the IB saturates. A solution is proposed to increase the validity range of this indicator.