The first isolation of Y. ruckeri in Portugal is reported. A year long microbiological survey of two trout farms (located on a river and a reservoir respectively) in N. Portugal, allowed us to isolate the Enteric Redmouth (ERM) bacterium from farmed fish and environment (water and sedimment). The bacterium was not detected in any of the wild fish tested. All the Y. ruckeri strains isolated from fish were serotype 01, whilst environmental isolates were serotype 01 and non-typeable strains. Biochemically, these strains differed in that the non-typeable isolates were able to ferment sorbitol. In addition, though all serotype 01 strains hydrolysed gelatin, some non-typeable isolates could not. Regardless of the serotype and source, two patterns of sensitivity to chemotherapeutants, differing only in their resistance to ampicillin, were found.