For 20 years, Federal law has mandated that low achieving children residing in low income areas receive supplemental services regardless of whether they attend public or nonpublic schools. Yet Chapter 1 participation rates have always been lower for nonpublic school children than they have for public school children. School size and problems of definition are two factors that account for this difference. This study, which examined the 39 larger districts in Massachusetts, explores the resulting variability after taking size and definitional problems into account. Among the conclusions are the following: (1) the absolute size of the Chapter 1 grant to a district appears to be a strong predictor of nonpublic school participation; (2) there is a strong correlation within districts between participation rates for public and nonpublic schools; (3) the same criteria for determining eligibility for Chapter 1 programs are used for students enrolled in public and nonpublic schools; (4) coordinators and nonpublic school principals viewed the relationship between the Chapter 1 program and the nonpublic schools as excellent; (5) having too few eligible students within a school was a problem faced more by out-of-district than within-district nonpublic schools; (6) the best measure of participation rate is the number of participants as a percentage of low-achieving Chapter 1 attendance area residents in participating schools. The characteristics of Chapter 1 students attending public and nonpublic schools are discussed. Tables and figures illustrate the data. Data sources are listed in an appendix. (BJV)