The taxonomic history of Helicoverpa armigera (Hübner) and Helicoverpa zea (Boddie) is discussed. Heliothis stombleri Okumura and Bauer is a new synonym of H. zea. The best diagnostic characters occur in the vesica of the male genitalia. These include (1) number of diverticula at the base, (2) length and number of coils, (3) number of cornuti visible on the uninflated vesica inside the aedoeagus, and (4) valve length. All diagnostic characters are described and shown. Methods for examining large samples of H. zea for possible H. armigera include placing the entire sample into suitable containers of 10% KOH for 18–24 h and dissecting the genitalia and measuring the valve length. It was found that H. zea had a longer mean valve length than H. armigera, but there was some overlap. Specimens whose valve length was <5.0 mm were examined for the diagnostic characters present on the vesica.