Variation of spontaneous somatic mutation frequency was studied in the stamen hairs of Tradescantia clone BNL 02 heterozygous for flower color (blue/pink; the blue color being dominant), one of the most stable clones in terms of spontaneous mutation frequency. Young inflorescence-bearing potted plants of this clone were grown under several different controlled environmental conditions, and the spontaneous pink mutation frequency in the stamen hairs was scored daily for three (partly two) weeks. The average number of hairs per stamen decreased as inflorescences became older, especially during the first one week or so after the inflorescences initiated flowering. The average number of cells per hair also decreased with the age of inflorescences, but the decreases were much smaller than those in the number of hairs. On the other hand, the spontaneous mutation frequency expressed as the number of pink mutant events per 104 hair-cell divisions did not show any significant changes with the age of inflorescences. The spontaneous mutation frequency varied, however, depending on the controlled environmental conditions and showing nearly significant negative and positive correlations with average temperature and diurnal temperature difference, respectively, although the variation observed was very much smaller than those reported earlier in a temperature-sensitive mutable clone KU 20 and also smaller than those in other clones.