The estimation of soil fertility cultivated with tobacco, as a result of superior yields, qualitatively and quantitatively, needs the extension of the investigation field offerred by agrochemical methods through the introduction of some indices of microbiological and enzymatical activity of the soil. If the biological total activity of the soil can be estimated through the dehydrogenase activity, the soil fertility can be summarized by the fertility index of the soil (Stefanic-Baok), as interaction of dehydrogenase and catalase activities. Experiments (1982-1987) in a crop rotation of 4 years with Oriental type tobacco (cultivars Djebel 123 and Djebal M 83) fertilized with chemical fertilizers and organic fertilizers, emphasized the fertilization with compost, 10-15 t/ha, applied directly to the tobacco, leading to the considerable increase in yield and to superior quality. The biological index of fertility, based on the dehydrogenase and catalase activities proved to be useful in the estimation of fertility level of the soil for the comparison of the influence of chemical fertilization with the organic fertilization in crop rotations of long duration with tobacco.