Education policy should be informed by research that distinguishes school-based learning from learning that occurs during non-school time. American studies find that socio-economic disparities in learning tend to widen over the summer months, the longest continuous stretch of non-school time. This paper analyzes the first large-scale Canadian study of summer learning. We collected data on literacy growth for a non-random sample of 1,376 Ontario children in Grades 1–3 during the summers of 2010 and 2011. Summer learning was widely dispersed with a mean of zero, and equal proportions of children had substantial learning gains and losses. There were strong disparities by family socio-economic status (SES), as affluent children gained literacy while those from poorer families lost literacy. We attributed 25 percent of the gap between the top and bottom SES quartiles at the start of the school year to the previous summer. We discuss implications for Canadian education policy and future research.