Plasmids belonging to the IncU incompatibility group are mobile genetic elements isolated frequently from Aeromonas spp. These plasmids share structural and functional characteristics and often carry Class-1 integrons bearing antibiotic resistance genes. In this work the ability of two IncU plasmids, pAr-32 and pRAS1 to establish in different A. hydrophila strains after conjugal transfer was studied. In vitro transfer frequencies on solid surface ranged from 10(-1) to 10(-6) for pAr-32 and from 10(-3) to 10(-5) for pRAS1. While carrying out these experiments we detected four strains unable to acquire plasmid pRAS1, indicating that the genetic background of recipients affects the establishment of the plasmid. We explored the possible reasons why these strains failed to yield transconjugants after mating experiments using A. salmonicida 718 as a donor. Factors included donor cell recognition, incompatibility, surface exclusion and restriction of incoming DNA. We found that none of these factors could explain the refractivity of non-receptive A. hydrophila strains to yield transconjugants. Although we do not know the reasons of this refractivity, we may speculate that these isolates lack a product necessary to replicate or stabilize plasmid pRAS1. Alternatively, these strains could contain a product that impedes plasmid establishment.