Antisense noncoding transcripts, genes-within-genes, and convergent gene pairs are prevalent among eukaryotes. The existence of such transcription units raises the question of what happens when RNA polymerase II (RNAPII) molecules collide head-to-head. Here we use a combination of biochemical and genetic approaches in yeast to show that polymerases transcribing opposite DNA strands cannot bypass each other. RNAPII stops but does not dissociate upon head-to-head collision in vitro, suggesting that opposing polymerases represent insurmountable obstacles for each other. Head-to-head collision in vivo also results in RNAPII stopping, and removal of collided RNAPII from the DNA template can be achieved via ubiquitylation-directed proteolysis. Indeed, in cells lacking efficient RNAPII polyubiquitylation, the half-life of collided polymerases increases, so that they can be detected between convergent genes. These results provide insight into fundamental mechanisms of gene traffic control and point to an unexplored effect of antisense transcription on gene regulation via polymerase collision.