Following the spread from its origins in China, Asian lineage highly pathogenic avian influenza H5N1 was first recorded in Europe in Turkish poultry and in poultry and wild birds in Romania in early October 2005. On 19 October 2005 Croatia became the second European country to record an outbreak in wild birds, involving Mute Swans Cygnus olor. Subsequent surveillance in Croatia revealed further instances of H5N1 in dead and sick Mute Swans in 2005, and in 2006 in more dead Mute Swans and in living, apparently healthy, Black-headed Gulls Larus ridibundus, but not in poultry. The observations presented here suggest that Croatia experienced two independent incursions of Asian lineage HPAI H5N1, but genetic confirmation is not available. Potential routes of introduction by wild birds, the poultry industry and fish-farming practices are discussed, but the evidence for all of these remains circumstantial.