Mothers' spontaneous teaching of their 4-year-olds was observed during block play in 230 dyads from low-income African American, Mexican, Dominican, and Chinese backgrounds. Blocks contained graphics that could be used to teach concepts in literacy, math, construction, or body/color. Coders noted how often mothers taught each of the concepts, and whether they taught at basic ("This is an "A"") or advanced ("C-a-t spells cat") levels. Additionally, mothers' negative and positive affect and feedback were coded. Ethnic differences were seen in the concepts that mothers taught, the levels of their teaching, and the amount and valence of mothers' feedback and affect towards their children. Within immigrant families, English-language use was positively associated with amount of teaching, engagement in advanced teaching, and feedback to children. The sociodemographic characteristics of families shape parents' structuring of learning opportunities for children and in turn influence children's developing skills and school readiness. (Contains 3 tables, 2 figures, and 1 footnote.)