The relation of crossing over to post-diplotene association of chromosomes in hybrids between Zea mays and Euchlaena mexi-cana has been studied. The data show clearly a relation between the two. When crossing over is reduced, post-diplotene chromosome association is likewise reduced. This relation supports Darlington's hypothesis that chiasmas are responsible for post-diplotene chromosome association. The data reported are not necessarily inconsistent with any of the theories of crossing over put forward by Belling, Sax and Darlington.