The aim of this longitudinal study was to further ourunderstanding of the reasons for social classdifferences in growth of decoding and readingcomprehension skills from beginning kindergarten throughchildren's fourth grade year. Participants wereenrolled in five public schools in a moderately sizedsouthern American city (n = 197). We examined ifbeginning kindergarten levels of three kinds ofreading related abilities explained social classdifferences in growth of reading skills during thetime periods of beginning kindergarten to children'sfirst-, second-, third-, and fourth-grade years. Thereading related abilities were phonological awareness,rate of access to phonological information inlong-term memory, and print knowledge. We found thatthe reasons for social class differences in growth ofreading skills depended on the time interval that wasconsidered. During the earliest time interval, socialclass differences in growth of decoding skills werecompletely accounted for by performance on the controlmeasures of general verbal intelligence and prior wordreading skills. During the remaining time periods,social class differences in growth of decoding andreading comprehension skills persisted whenperformance on the three kinds of reading relatedabilities and the control measures were accounted for. The greatest attenuation of SES differences in growthof reading skills occurred when beginning kindergartenlevels of print knowledge were taken into account.