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The parasitoid wasp Trichogramma kaykai with a haplo-diploid sex determination has a B chromosome called the paternal sex ratio (PSR) chromosome that confers paternal genome loss during early embryogenesis, resulting in male offspring. So far, it is not well known whether the PSR chromosome has unique DNA sequence characteristics. By compa- rative AFLP fingerprinting of genomic DNA from wasps with and without the PSR chromosome, we isolated DNA from PSR-specific bands. Fourteen of such DNA fragments were analysed to confirm their PSR specificity. Seven were se- quenced and two (PT-AFLP 1 and PT-AFLP1 3) were identified as parts of retrotransposon genes based on BLAST searches. Internal primers designed from a third AFLP fragment allowed PCR amplification of a PSR chromosome specific marker, which can be used to screen for the PSR trait in male wasps. Southern analysis revealed a dispersed repetitive na- ture of this third sequence in the T. kaykai genome, suggesting that it is part of a transposon. A fourth AFLP fragment (PT-AFLP 5) appears to be a large repetitive sequence on the PSR chromosome. This sequence is also found in the ge- nome of both T. kaykai and the closely related species Trichogramma deion, but its distribution on the PSR chromosome strongly resembles that of T. deion rather than that of T. kaykai. Our results provide further insight into the repetitive na- ture of sequences comprising B chromosomes and their similarities with their host and closely related species. Resume´ : La guepe parasitoo¨de Trichogramma kaykai presente un systeme de determination du sexe haplo-diploo¨de ouun chromosomes B, appelechromosome sexuel paternel (PSR), confere la perte du genome paternel au cours des premiers stades de l'embryogenese et resulte en une descendance male. Il n'est pas bien clair ace jour si le chromosome PSR pre ´- sente des caracteristiques uniques au niveau de sa sequence d'ADN. Par comparaison des empreintes AFLP obtenues avec l'ADN genomique provenant de guepes avec ou sans chromosome PSR, les auteurs ont isolel'ADN des amplicons specifi- ques au PSR. Quatorze de ces amplicons ont eteanalyses pour confirmer qu'ils etaient specifiques du PSR. Sept ont ete ´ sequencese t il aetetrouveque deux de ceux-ci (PT-AFLP 1 et PT-AFLP 3) correspondaient ades sequences geniques partielles de retrotransposons sur la base de recherches BLAST. Des amorces internes synthetisees apartir d'un troisieme amplicon AFLP ont permis d'amplifier un marqueur specifique du chromosome PSR, lequel permet d'identifier ce carac- tere chez les guepes males. Une analyse Southern a reveleque cette sequence etait repetee et dispersee dans le genome du T. kaykai, ce qui suggere qu'il s'agit d'un segment de transposon. Un quatrieme fragment AFLP (PT-AFLP 5) semble cor- respondre a une grande sequence repetee sur le chromosome PSR. Cette sequence est presente tant dans les genomes du T. kaykai que de l'espece voisine Trichogramma deion, bien que sa distribution sur le chromosome PSR ressemble davantage acelle observee chez le T. deion que chez le T. kaykai. Ces resultats apportent un eclairage additionnel sur la nature repe ´- titive des sequences constituant le chromosome PSR et leur similitude avec les sequences de l'hote et d'especes apparen- tees. Mots-cles : Trichogramma kaykai, chromosome PSR, chromosome B, AFLP, retrotransposon.