The study reported in this article examined relations between ethnicity, out-of-school activities, and arithmetical achievements in Latin American and Korean American children in first, second, and third grades. Three types of data were collected: parents' educational attitudes and beliefs, parents' reports of children's everyday activities with arithmetic and money, and children's performance on arithmetic tasks. Few differences emerged in parents' attitudes about education, although their reports revealed differences in children's out-of-school activities, with Latin American children more often engaged in instrumental activities with money, and Korean American children more often engaged in activities intended to support their school learning. Children's performances on arithmetic tasks mirrored their engagement in out-of-school activities: Latin American children solved correctly more tasks with money than denominational chips, and Korean American children solved correctly more tasks with chips than money. The results are discussed with respect to understanding connections between culture and children's developing mathematical understanding, and creating culturally relevant school instruction that builds on students' informal mathematics knowledge. (Contains 5 tables, 6 figures and 7 footnotes.)