The mechanism of cold sensitivity of Aspergillus fumigatus ON5, a 37 degrees C-sensitive mutant derived from A. fumigatus I-21 (ATCC 32722) by five sequential mutations, was investigated. The rate of in vivo protein synthesis by ON5 was not affected for 2 h following a shift from 45 to 34 degrees C, but the rate of in vivo RNA synthesis dropped almost immediately. The RNA polymerases 2 h following a shift from 45 to 34 degrees C, but the rate of in vivo RNA synthesis dropped almost immediately. The RNA polymerases of ON5 possessed wild-type activity in vitro at a nonpermissive temperature (34 degrees C) indicating that the reduction in the rate of in vivo RNA synthesis did not result from cold sensitivity in transcription, but was possibly a result of rapid feedback inhibition of transcription. Mutant ON5 was not able to produce ribosomes at a nonpermissive temperature as evidenced by the fact that no 3H-labelled amino acids were incorporated into the monosome, large ribosomal subunit, or small ribosomal subunit at 34 degrees C. Ribosomal subunit assembly or ribosomal RNA processing appears, therefore, to be the cold-sensitive cellular function in ON5.