Hydrogen cyanide production by two pathogenic isolates of an unidentified basidiomycete was studied in shake culture in synthetic and natural media. Both isolates release HCN in infected host plants but one isolate (type A) failed to produce HCN in culture. Curves for growth and HCN production by the second isolate (type B) indicated that HCN is a product of autolysis and is not dependent on the provision of specific substrates in the medium.