Focusing on the extent to which high school students are interested in their free-choice books and on the relationship between the students' reading level and the readability level of the chosen books, a study was devised to test the validity of Edward Fry's Readability Principle. This principle states that high motivation can overcome high readability, but low motivation demands low readability. Reading levels of the students were obtained from scores on standardized tests, and the Fry Readability Scale was used to calculate the readability of the books. The 58 students involved in the study selected a total of 75 books and completed 56 of them. Each of their selections constituted either a positive or a negative match. If the reading level of the student equalled or exceeded the readability of the book, the match was positive. If the reading level of the student was below the readability of the book, then the match was negative. There were 43 positive and 32 negative matches. In individual interviews, the students reported the number of pages they had read in each book and indicated their level of interest in each. It was found that students with high interest, in both positive and negative groups, read more pages than those with low interest. Little or no relationship was found between students' reading levels and readability levels. The study confirms the Fry Readability Principle. (FL)