Last name 小柴 (Kosiba 
, 小柴 
, Косиба 
) , worldwide, is quite a rare surname. The surname 小柴 is characteristic of Eastern Europe, particularly Poland, where it is quite a rare surname, Slovakia, and Hungary. Measured by absolute frequency, it is commonest in Poland, the United States, and Brazil. Also, 小柴 is the forename as well as the last name.