Abstract Spontaneous mutation rates to streptomycin (SM) resistance were measured for dormant B. megatarium spores and for spores at different times into germination. The number of SM-resistant colonies which formed increased with time into germination, although there was no simultaneous increase in the size of the population. Since the increase in the number of mutations seems to parallel the expected onset and rate of DNA synthesis, these results appear to show that SM resistance is genetically dominant over SM sensitivity. This interpretation contradicts the widely accepted evidence from E. coli that for mutations on the genome SM resistance is recessive to SM sensitivity. It is known, however, that E. coli can carry an episome which codes for a response to SM. At this locus, SM resistance is dominant. Although no episome has been reported for a Bacillus, our results could be explained by assuming that one exists and that the genetics governing its response to SM are like those of E. coli.