The aim of this paper is twofold: first, we want to explore the intra-firm diffusion of information and communication technologies (ICT) within the Tunisian firms and to characterize its general trends of adoption and usage. Second, we want to emphasize the rank and epidemic effects stressed by the disequilibrium models of intra-firm diffusion of innovation following the traditional view of (Mansfield, 1963, Antonelli 1985). Based on face-to-face questionnaires of a random sample of 175 firms our paper shows that: (i) three technological waves of ICT adoption are well characterized in the Tunisian manufacturing sector. This dynamic of adoption is linked to the age of the technologies. Time is the main explanatory variable for intra-firm diffusion of these technologies. (ii) A positive correlation between the size of the firm, seniority and the depth of adoption is found. These econometric estimates show that the rank effect is well characterized within the Tunisian firms. (iii) A positive correlation between technological absorptive capacity building and intensity of ICT usage is found. This correlation confirms the epidemic effect. (iv) Our results show that disequilibrium models' explanations of intra-firm diffusion of innovation are valid within the Tunisian manufacturing sector and seem more appropriate than the equilibrium theory for developing countries.