1. 1. A completely synthetic medium was found for Escherichia coli Q-13 (ribonuclease I-less mutant), containing methionine, tryptophan, tyrosine and leucine. 2. 2. Ribosomal RNA degradation was observed in Q-13 in phosphate- and magnesium-deficient states. All the patterns coincided well with those in the wild type of E. coli. 3. 3. When pulse-labelled with [ 14C]uracil, RNA gave [ 14C] UDP and [ 14C]UMP as degradation products in the presence of Na 2HPO 4. When labelling with [ 14C]adenine was used, a very small amount of [ 14C]ADP was detected among the degradation products. 4. 4. E. coli Q-13 contains higher nucleoside diphosphate pyrophosphatase activity than the wild strain. Evidence is given that it masked the polynucleotide phosphorylase (EC 2.7.7.8) actually present in E. coli Q-13. 5. 5. It is proposed that the enzymes responsible for the degradation of ribosomal RNA and messenger RNA are polynucleotide phosphorylase and ribonuclease II (EC 3.1.4.1.) in both the wild and mutant strains used.