Previous studies have suggested that the size of budding yeast cells is controlled almost entirely by a cell size checkpoint that acts in G1 phase. In contrast, we show here that a previously unknown checkpoint that acts during mitosis plays a major role in cell size control. Over 80% of cell growth in rich nutrient sources occurs during mitosis, while only 8% occurs during G1 phase. When growth is slowed by poor nutrients, the mitotic checkpoint compensates by prolonging both metaphase and anaphase, thereby increasing the duration of growth. The mitotic delay in poor nutrients is controlled partly by inhibitory phosphorylation of Cdk1 and partly by a novel mechanism that is independent of Cdk1 inhibitory phosphorylation. PP2A associated with the conserved Rts1 regulatory subunit is a key component of the checkpoint and appears to control cell size by enforcing a mechanistic link between growth rate and size.