A virus with spherical particles c. 28 nm in diameter was sap-transmitted from different cassava (Manihot esculenta) cultivars to a limited range of species in the families Chenopodiaceae and Solanaceae. Cassava seedlings infected by inoculation with sap or with purified virus preparations did not show any symptom, although the virus was readily detected by ELISA or by further inoculations. Leaf extracts from infected Nicotiana benthamiana were infective after dilution of but not 10-4, and after heating for 10 min at 70°C, but not at 72°C. The virus was purified from N . benthamiana, N. clevelandii or from cassava. On sucrose gradients, the virus particles sediment as three components all containing a protein of mol. wt c. 57 OOO. The genome of the virus is composed of two RNAs of mol. wt c. 2.54 x lo6 (RNA-1) and 1.44 x 106 (RNA-2). RNA-2 was detected in the middle and the bottom nucleoprotein components, and RNA-1 only in the bottom component. An antiserum prepared to purified virus particles was used to readily detect the virus in cassava and other host plants by ELISA and by ISEM. No serological relationship was shown between this virus and eight nepoviruses, including the recently described cassava green mottle nepovirus infecting cassava in the Solomon Islands (Lennon, Aiton & Harrison, 1987). The virus described here is the first nepovirus isolated from cassava in South America, and is named cassava American latent virus.