Phytomelanin in the pericarp of sunflower,Helianthus annuusL., achenes is an effective mechanism for reducing yield losses due to achene injury by larvae of the sunflower moth,Homoeosoma electellum(Hulst). In field trials, a highly significant negative correlation (r = –0.93) existed between percent injured achenes and percent achenes having phytomelanin in the pericarp. We showed in penetrometer studies that pericarps with phytomelanin harden much more quickly than pericarps lacking phytomelanin, and that there are genotypic differences for its expression among lines having the trait. We demonstrated in pericarp developmental studies that phytomelanin begins to be deposited between the hypodermis and sclerenchyma of pericarps by day 3 after ovule fertilization, and that its deposition progresses rapidly through day 13 after ovule fertilization. In laboratory feeding bioasays with whole achenes and pericarp mash incorporated in a wheat germ diet, we showed that phytomelanin exhibited both antizenosis and antiobiosis toward larvae ofH. electellumwhen eaten.