Bacillus subtilis strain PB 2427 temperature sensitive in the synthesis of RNA during spore germination and outgrowth has been characterized to some extent. At non permissive temperature (46 degrees C) strain PB 2427 synthesizes stable and unstable RNA for 50 min from the beginning of germination and then stops. Most of the stable RNA is degraded to shorter molecules but can be identified as ribosomal RNA by hybridization-competition experiments. At non permissive temperature, in the presence of chloramphenicol, synthesis of RNA proceeds, though at a reduced rate, for at least 90 min. By hybridization-competition experiments it can also be shown that the RNA synthesized at 46 degrees C in the presence of chloramphenicol includes transcripts that are absent, from the RNA synthesized at 46 degrees C in the absence of drug. The RNA polymerase (holo and core) purified from vegatative cells of the mutant strain does not appear to have a greater heat-lability as compared with the enzyme purified from the parental strain. At non permissive temperature only six polypeptide chains with MW ranging from 47,000 to 78,000 daltons are synthesized by the germinating spores of the mutant.