Over the course of their domestication, crop plants have frequently undergone reduction or loss of chemical defenses against herbivores and pathogens. However, in some crops, some defenses have persisted and they appear to play diverse, but often little-understood, roles in crop ecology. Crop species that are polymorphic for chemical defenses offer unique opportunities for exploring the roles of crop chemical defenses in agroecosystems. In these coupled human and natural systems, the level of defense has consequences not only for the plant but also for how farmers organize their activities. Understanding variation in chemical defense thus requires the analysis of costs and benefits of defense not only to the plant but also to the farmers. I will examine the biocultural ecology of chemical defense in three crop plants characterized by such polymorphisms : sorghum (Sorghum bicolor, Poaceae), in which the concentration of tannins in seed coats varies greatly among varieties ; oca (Oxalis tuberosa, Oxalidaceae), which has sweet and sour varieties differing in their content of oxalic acid ; and manioc or cassava (Manihot esculenta, Euphorbiaceae), in which there exist both non-toxic sweet and highly toxic bitter varieties, reflecting enormous variation in the cyanogen content of the plant's tuberous roots. In all these plants, variation in chemical defense is central to the coevolution of plants and cultural practices.