Five biotypes of the citrus nematode, Tylenchulus semipenetrans Cobb, are reported in the literature. Four of them occur in California (Baines et al., 1974) one being capable of active reproduction on olive trees (Lamberti and Baines, 1970). A fifth, in Florida, is said by Stokes (1969), to parasitize the grass Andropogon rhizomatus Swallen but not citrus rootstoks. The results of two experiments, to characterize as biotypes three Italian populations of T. semipenetrans, are discussed in this paper. One experiment was done in a glass-house at Bari and the other in a lath-house at Catania, in 1972-1973, to test the infectivity and the pathogenecity of the nematode on citrus, olive and grape plants (Lamberti et al., 1974 and 1976).
