Developmental theories and previous research have emphasized the significance of cooperation and self-control in middle childhood. The present study extends previous research by examining (a) the growth of cooperation and self-control as well as the relations between them in middle childhood (third to sixth grade) and (b) the extent to which mothers' and fathers' parenting during early childhood (54 months and first grade) was associated with children's cooperation and self-control. The sample included 705 children (51% female, 86% White) and their mothers, fathers, and teachers in the National Institute of Child Health and Human Development (NICHD) Study of Early Child Care and Youth Development (SECCYD). Children, on average, exhibited increases in self-control but not cooperation from third to sixth grade though the increases were smaller for children who had higher self-control or cooperation at third grade. Children who exhibited higher self-control at third grade tended to exhibit higher cooperation at third grade; similar positive associations emerged for the changes in self-control and cooperation over time. In addition, if a child exhibited higher self-control at one time point relative to their typical average level, they tended to also exhibit higher cooperation at the same time point relative to their typical average level. However, these relative deviations within person were not associated over time. Lastly, maternal and paternal sensitive and stimulating parenting in early childhood was positively associated with children's cooperation and self-control in middle childhood. Overall, our findings shed light on the growth of and the relations between cooperation and self-control in middle childhood and highlight the role of maternal and paternal parenting in early childhood. (PsycInfo Database Record (c) 2021 APA, all rights reserved).