Interspecific hybridization leading to viable andfertile fishes in the Tilapia group (Pisces, Cichlidae) is readily obtainedin stock populations and occasionally occurs in natural populations. Tilapias thus provide a valuable model to investigate hybrid genome evolution in vertebrates. As part of an aquaculture development project, (Oreochromis niloticus x Sarotherodon melanotheron) hybrids were produced and propagated for three generations. As evidenced from karyotypic analysis, both parental species are endowed with a typical Tilapiini chromosomal formula (2n =44 ST) which was found to be conserved in hybrids. In order tocharacterize the structure ofthe 2 specific genomes classical cytogenetic methods (Chromomycin A3 staining, C-banding) as weIl as FISH (telomeric, ribosomal, and rex retro-transposons probes) were achieved. Furthermore we performed comparative genomic hybridization (CGH) in offspring of hybrids and pure specimens using genomic DNA from O. niloticus and S. melanotheron. CGH revealed a partial painting restricted to heterochromatic regions also characterized by positive C-banding and retro-transposon rich regions (i.e. the centromere and the short arms of aIl chromosomes and especially the distal part of the long chromosome arm of the first pair). Despite cross-hybridization of the probes we were able to trace each centromere back to the parental species and to show the occurrence ofrecombination events in thefirstchromosomepair. Moreoverthe firstlong pair was expected to be sex linked according to the study of synaptonemal complexes. This idea was supported by the results of CGH and especially FISH of transposons but the size heteromorphism ofthe long arm in the first pairwas 0 bserved in sorne males and sorne females as weIl. These results suggest that, although species exhibit an important eco-morphological diversity, genome organization is highly conserved among the Tilapiini apart sorne differences in heterochromatic structure and pattern of re