The present study relates a within-family variable, birth order position, to acculturation among second and third generation Japanese Americans. The study tests a general hypothesis that firstborn (including only children) will be less assimilated to the dominant culture than their siblings in families where the parents are trying to maintain their identity with the culture of origin. This hypothesis was generally supported. Compared to laterborn Nisei, firstborn Nisei were less assimilated. For example, firstborn Nisei were more likely to live in Japanese American neighborhoods, to use and learn Japanese language, to have stronger Japanese family values, and to be Buddhist or Shinto. Likewise, compared to laterborn Sansei, firstborns seemed to be more informed and interested in Japanese values and culture and to have more culturally traditional perspectives. Results support that, in general, compared to laterborns, firstborns in both generations were more traditionally "Japanese". Implications of these findings for understanding acculturation are presented.

Key words: acculturation, assimilation, birth order, Japanese