This paper examines the daily labor supply of a potentially important, but often overlooked, source of instruction in U.S. public schools: substitute teachers. I estimate a sequential binary-choice model of substitute teachers' job-offer acceptance decisions using data on job offers made by a randomized automated calling system. Importantly, this randomization generates exogenous variation in offer quality. I find that determinants of substitutes' offer-acceptance decisions include the offer's arrival time, commute time, day of week, classroom type, school type, and school quality. Interestingly, conditional on school quality student demographics do not significantly influence substitutes' decisions. Longer and higher paying full-day jobs are preferred to half-day jobs, although conditional on daily pay, job length does not significantly impact daily labor-supply decisions. Preferences for several job characteristics are found to vary with substitutes' regular-teacher certification status. Policy implications of these findings are discussed. (Contains 9 tables and 3 figures.)