Mutants of Myxococcus xanthus FBt unable to form myxospores in response to 0.5 M glycerol arise spontaneously with a frequency of 1--3 X 10(-5). These mutants are designated glc. Ultraviolet mutagenesis increases the frequency to a maximum of 7% of the survivors. The reversion frequency following ultraviolet irradiation of spontaneous glc mutants is less than 10(-3). Of four glc mutants examined, none form myxospores in response to the alternative inducers, ethylene glycol and dimethyl sulphoxide. One glc mutant is induced by 1.5 M glycerol; strain FBt responds to this glycerol concentration with low efficiency myxospore formation. Strain FBt and glc mutants all produce myxospores with low efficiency in response to phenyl ethanol. Of 117 glc mutants tested, 109 form fruiting bodies containing mature myxospores; thus, mutations to the glc phenotype do not normally block myxospore formation within the fruiting cycle of the organism.