The sex gap in language and reading was investigated. Studies which indicate that girls generally excel over boys in the language development and reading area were reviewed. Three educational levels were included in the investigation: preprimary, primary, and upper elementary. The author's study was made in the Los Angeles City Schools to determine whether boys' reading achievement would be affected by instruction in sex-segregated groups. At the end of the school year test scores indicated that girls, as a group, achieved significantly more than the boys and also showed significantly greater reading growth. Analysis of the data did not show that boys taught alone gained significantly more in achievement or in growth than boys taught in heterogeneous sex groupings. The study did, however, reveal seven basis areas of difference in the language-development patterns of boys and girls: verbal facility, listening skills, auditory discrimination, attention span, story preference, personality style and activity levels, and goals and motivations. Six years of subsequent research supported these findings. References are included. (Author/NH)