A genetic variant virulent on tomato was selected by passaging field isolates of Colletotrichum gloeosporioides, obtained from mango, at least twice through wounded tomato fruits. The field isolates and isolates virulent on tomato showed ribosomal DNA restriction fragment length polymorphisms (RFLP). They also had different random amplified polymorphic DNA (RAPD) banding patterns. The mango field isolates produced conidia of sizes ranging from 12 to 22 μm and approx. 5% of these conidia were multinucleate. Cultures of the genetic variant produced uniformly-sized conidia (approx. 14 μm) which were uninucleate. The genetic variant was isolated in culture, only with difficulty, as a single conidium (L2t3) from a culture (L2) which, in turn, was grown from a single approx. 20 μm conidium obtained from one of the mango field isolates (M11). It is suggested that two genetically different nuclei residing within individual multinucleate conidia gave rise to the observed variation. The importance of heterokaryosis in relation to the considerable genetic variation and apparent adaptability seen in C. gloeosporioides is discussed.