Experiments were conducted to characterize further the biochemical defects of crib-1 (PJ30201), a coldsensitive mutant strain of Neurospora crassa with a defect in ribosome biosynthesis. The results are as follows. (i) The critical temperature for the expression of the mutant growth and ribosome phenotypes is in the range of 18 to 20 C. (ii) No preferential breakdown of 37S cytoplasmic ribosomal subunits synthesized by crib-1 at 25 C occurs after a shift to 10 C. (iii) Ribosomal subunits synthesized by crib-1 at 25 C function normally in in vivo protein synthesis at 10 C. (iv) Whereas wild type synthesizes both ribosomal subunits in a coordinate manner after either a temperature shift-down (25 to 10 C) of a shift-up (10 to 25 C), noncoordinate synthesis of ribosomal subunits owing to underproduction of 37S subunits occurs in the crib-1 strain immediately after a temperature shift-down. (v) After a shift from 10 to 25 C crib-1 exhibits a 12-h lag before the growth rate and the rate of synthesis of 37S subunits begin to increase significantly. (vi) At 10 C crib-1 synthesizes unequal amounts of 25S and 17S ribosomal ribonucleic acid (rRNA) molecules, resulting from a greatly reduced accumulation of stable 17S rRNA. The mutant phenotypes of crib-1 are proposed to be the result of a defect in rRNA processing.