To gather longitudinal student achievement data from schools utilizing the four-day week, Iowa Tests of Basic Skills (ITBS) grade equivalent scores were obtained in 1984 from five rural Colorado school districts for four consecutive years--two years prior to and two years following the districts' change to the shortened week. A first analysis identified two groups of students: one (N=62) provided a sequence starting with grade three and going through grade six, and a second (N=45) started with grade four and went through grade seven. A second analysis examined the performance of a given grade level for each of the four consecutive years for grades three, four, five, and six. Analysis of the ITBS scales revealed that the change to a four-day school week had no effect on student academic achievement, although there was a suggestion that there might be some leveling of performance during the first year the schools were on the four-day schedule. Analysis of the same grade level across the four-year period also provided no clear evidence that the change had any effect on student achievement. The significant differences might be explained by the observation that one year or class of students performs quite differently from another. (BRR)