Abstract: In a trial to evaluate the efficacy of arbuscular mycorrhizal (AM) fungi to control Pythium damping-off of cotton, an increase of soil population of AM fungi was achieved by growing a mixture of trap crops for several months whereby a total count of 996 spores/100 g soil was achieved. As expected, plants grown in soil with increased mycorrhization showed a prominent positive change in all growth parameters by comparison with the non-mycorrhized-control soil. The results revealed also that AM fungi induced a significant reduction of disease incidence in infested soil from 68 % to 29% in pots and from 77.3% to 50.7% in the field. However, such values of reduction in disease incidence although mathematically significant, are not economically acceptable and make the reliability of AM alone, as a control mean of cotton damping-off by Pythium, questionable. Justifications of such limitation are given and suggestions of improvement of efficacy of AM fungi are presented. The systemic fungicide mefanoxam, by comparison with mycorrhiza, showed up a high degree of efficacy by reducing the disease incidence down to 12% in pots and 9.6% in field. Key words: Arbuscular mycorrhizal (AM) fungi, Pythium aphanidermatum, damping-off, cotton, biocontrol