|Abstract||A major gene for rust resistance has been identified in cultivar R570 (Daugrois et ai, 19%). Rust disease is caused by the fungus Puccinia malenocephala Syd P. Syd and is distributed worldwide. Inheritance of resistance to rust was investigated in the self progeny of cultivar R570, which was also used to build the RFLP genetic map of this cultivar (Grivet et ai, 1996). Resistance was evaluated through both field and controled greenhouse trials. The results demonstrated that the field scores are very well correlated between plant cane and first ratoon and in very good agreement with the greenhouse artificial inoculation tests. A cle;r-cut 3 (resistance) : 1 (susceptible) segregation indicative of a dominant resistance gene was observed. RFLP mapping indicated that this gene was linked at approximately 10 cM to a marker revealed by the cDNA clone CDSR 29. This marker was not linked to any of the cosegregation groups of the R570 map (Grivet et ai, 19%). This gene is the first major gene identified in sugarcane. ICSB was thus interested in using it to test the feasability of map-based cloning in the complex aneuploid polyploid that sugarcane is. For this purpose the BAC library developed by RWing and co-workers was constructed using cultivar R570 (Tomkins et ai, 1999). The first step for map-based cloning is to develop a fine map in the vicinity of the gene; this was the objective of the present project. For this purpose, we developed and characterized for rust resistance large selfed progenies of R570 and we followed two strategies to find markers closely surrounding the rust resistance gene ; one is based on the synteny and colinearity demonstrated between Graminae species and the second is based on Bulked Segregant Analysis (BSA) with AFLP markers. We also tested the pathogenicity variability of the fungus.