This paper addresses the segregation of student populations that results from adoption of multitrack year-round school attendance calendars. Its focus of analysis is the link between student achievement and the use of a multitrack year-round education (YRE) program to structure student attendance. Data for this study consist of achievement test data from the spring 1998 administration for a large urban California school district for grades 2 through 6. Results of statistical analysis of the data show that there are two forms of opportunity segregation related to multitrack YRE in this district. First, YRE schools have lower achievement and more challenging student populations (that is, higher proportions of students with characteristics associated with lower achievement) than do traditional calendar (TC) schools. The second form of opportunity segregation is between tracks at YRE schools; it changes over time, solidifying and elaborating intertrack differences. The critical driving force behind this demographic, resource, and achievement segregation is parental choice. Parental political pressure to use vouchers to create learning opportunities for their students in multitrack YRE learning environments had contrary outcomes. At present, there is no compelling evidence that multitrack YRE education programs lead to improved student achievement. (Contains 43 references.) (RT)