A single-chained antifungal protein with a molecular weight of 6.5 kDa and displaying a novel N-terminal sequence was isolated from dried juvenile cicadas which are used in traditional Chinese medicine, by using ion exchange chromatography on DEAE-cellulose, affinity chromatography on Affi-gel blue gel, ion exchange chromatography on SP-Sepharose and then gel filtration on a Superdex peptide column. The peptide, designated cicadin, exerted potent antifungal activity with IC(50) values at nonomolar concentrations against a variety of fungi including Botrytis cinerea, Mycosphaerella arachidicola, Fusarium oxysporum, Rhizoctonia solani and Coprinus comatus. Cicadin suppressed the activity of HIV-1 reverse transcriptase and stimulated the proliferation of murine splenocytes.