One of the most important characteristics of a qualified high school teacher is college training in the subject in which he or she teaches. Data from the 1990-91 Schools and Staffing Survey (SASS)--conducted by the National Center for Education Statistics to collect information on teachers' undergraduate and graduate degrees and their school teaching assignments--were examined to determine the percentages of out-of-field teachers in core subjects and whether differences exist among schools in the extent of out-of-field teaching assignments. Results indicate that in 1990-91: (1) relatively large percentages of high school teachers taught at least one core course out of field; (2) greater percentages of teachers in private high schools taught out of field in mathematics, science, social studies, and English than did their counterparts in public high schools; (3) public and private high schools with the lowest enrollments had greater percentages of teachers teaching out of field than did the largest schools; (4) public high schools with 50 percent or more students receiving free or reduced-price lunches had higher levels of out-of-field teaching in selected subjects than did schools with less than 20 percent of these students. Results indicate that out-of-field teaching in core subjects is common in both public and private U.S. high schools, but is not due to lack of basic training on the part of teachers. Additional analyses of SASS data are needed to examine the recruitment and hiring practices and conditions of schools with high versus low rates of out-of-field teaching. (HTH)