The viability of urediniospores of Melampsora medusae decreased markedly when stored in mixture with the conidia of Cladosporium aecidiicola. The rate of decrease was more rapid in samples stored at 16°C with high concentration of Cladosporium (ratio of urediniospores to conidia 10:1 by weight) than at −12°C and low concentration (20:1 by weight). Copyright {\textcopyright} 1980, Wiley Blackwell. All rights reserved