The discovery of a previously unknown isotype of Opuntia vilis Rose, collected by FE Lloyd in 1907 in the state of Zacatecas, Mexico, is reported in the collection of the herbarium of the New York Botanical Garden (NY). This isotype was probably donated to NY by the United States National Herbarium (US) judging by an annotation label included on the voucher. The isotype is composed of seven flower sections, one detached pistil, and one black and white close-up photograph of a plant in habitat. The existence of this isotype has been unnoticed in the literature, probably because the voucher was deposited in the general collection of NY together with other plant material, instead of in a separate type collection. The only annotation mark in the voucher was the blue ink stamp of Britton and Rose for their examination of The Cactaceae (Britton and Rose 1919).