Phenotype and magnification of two bobbed mutations allelic to the XNO region were studied at various temperatures. Results showed that magnification increases with increased severity of bobbed phenotype in that the reversion of the bbT6 allele is the same at high and low temperature whereas the thermosensitive bbP5 allele shows increased reversion at high temperature. Temperature shift experiments carried out during the development of premagnified males show that magnification which is followed by selection stage occurs during embryogenesis.