The fungus Fusarium oxysporum Sch.:Fr. presents a high biological and genetic variability, manifested by the existenceof many specialized forms and races. With the goal of evaluating the genetic relationship among different specializedforms, an experiment was carried out involving the application of two types of molecular markers. One consistedof sequencing the internal transcribed spacer (ITS) region of the ribosomal DNA of 17 isolates belonging toseven forms: F.o. melonis (4 isolates), F.o. dianthi (2), F.o. niveum (4), F.o. lycopersici (2), F.o. radicis-lycopersici (3),F.o. lagenaria (1) and F.o. luffae (1). Analysis of these sequences revealed that isolates from different forms presentan identical sequence while isolates of the same form appear distributed in different groups of the dendrogram. Thisobservation was confirmed by an AFLP analysis. In addition to the previously cited forms, ten more were studied inthis experiment, belonging to F.o. ciceris, F.o. cucumerinum, F. proliferatum and F.o. asparagi, as well as two non-pathogenicisolates. The dendrogram calculated with the AFLP markers did not reveal any genetic structuration of the10 specialized forms. These data, in line with those obtained by other authors, seem to suggest that in general, the specializedforms of F. oxysporum do not constitute monophyletic lineages because they evolve in a divergent way. It ratherseems that widely different genotypes could share similar genetic factors conferring pathogenic specificity.