The purpose of this study was to examine second graders' (n = 680) changing spoken nonmainstream American English (NMAE) use in relation to their oral language and reading comprehension achievement. Fall NMAE production was negatively associated with fall achievement scores. NMAE production generally decreased from fall to spring. Students who qualified for the US Free and Reduced Lunch program (FARL) and who had stronger language skills were more likely to decrease their NMAE use (i.e., dialect shifting) than their peers who did not qualify for FARL or their peers with weaker language skills. Dialect shifting for a sub-sample of 102 students who used substantial amounts of NMAE at the beginning of the school year was predicted by school context, controlling for reading and language skills--in general, students who attended more affluent schools dialect shifted to a greater extent than did their peers who attended higher poverty schools. Greater dialect shifting in this group predicted gains in reading comprehension from fall to spring.