The four organic acids; Gallic acid (GA), Jasmonic acid (JA), Oxalic acid (OA) and Tannic acid (TA); (used in three concentrations for each acids) were evaluated in vitro and in vivo for suppression of Rhizoctonia solani caused damping-off on cotton. In vitro study showed that JA and OA were ineffective in inhibiting linear growth of the fungus, while GA and TA showed antifungal activities. In non-infested soil, none of the tested acids caused phytotoxicity under greenhouse conditions for seedlings of cotton cultivar Giza 95 when they were applied as soil drench. All the organic acids caused significant increases in plant height and dry weight of seedlings. In infested soil, GA was effective in controlling damping-off disease only at the low concentration (25.0 mM). All the applied concentrations of the other acids were effective in controlling the disease.  Plant height and dry weight were not affected by any acid.  The effects of organic acids on peroxidase, polyphenol oxidase, free phenolic compounds and total phenolic compounds were variable depending upon the applied concentration.