Since the mid-1960s, many state governments have introduced subsidies for school districts that offer kindergarten. This paper uses the staggered timing and age targeting of these grants to examine how the childcare subsidy implicit in public schooling affects maternal labor supply. Using data from five Censuses, I estimate that four of ten single mothers with no younger children entered the work force with public school enrollment of a five-year-old child. No significant labor supply responses are detected among other mothers with eligible children. Results also indicate that at least one in three marginal public school enrollees would have otherwise attended private school. (Contains 8 tables, 2 figures and 43 footnotes.)