The number of secondary vibrissae in the mouse is normally 19. The tabby gene reduces this to about half and causes a marked variability of vibrissa number. Selection on tabby mice for increase and decrease in vibrissa number has been successful. These selection lines were maintained in segregation for + and Ta alleles, and this selection, practised solely on tabby mice, has resulted in both an increase and decrease in vibrissa number in normal mice.