Following the 2004 tsunami, a high incidence of an unknown epizootic infection was noted in 250 fallen sea fans which were recovered and reattached to substrate at Mu Ko Similan National Park. Tissue samples from the gorgonian Annella sp. were collected for microbial analysis in October 2005 when 56% of the sea fans were infected or already dead. The infection was characterised by white matter spreading over the axial skeleton leading to tissue disintegration. Aspergillus sp. and a further seven fungal genera were found to be present. A preliminary study of the Aspergillus sp. ITS1–5.8 S–ITS2 sequence showed highest relatedness to Aspergillus versicolor but lower relatedness to Aspergillus sydowii. A. sydowii causes sea fan mortality in the Caribbean (Fabricius and Alderslade 2001; Nagelkerken et al. 1997). Many infected sea fans in this study succumbed to the disease, but some colonies recovered and produced fresh tissue growth. Aspergillus sp. is a terrestrial soil fungus and it is possible that spores enter the sea from land runoff. Why it infects and kills sea fans is unknown. After the tsunami, water runoff was very high. The anatomy of the sea fan tissue, with an extensive network of solenia connecting polyps through the mesoglea (Fabricius and Alderslade 2001), may aid the spread of infection. Stressed gorgonians may also lose their anti-fouling protection, making them more susceptible to infection (Fabricius and Alderslade 2001). Weakening of the sea fan immune system by pollution and rising sea temperatures is one hypothesis which has been advanced to explain the increased prevalence of Aspergillosis. This is the first time Aspergillosis has been shown to be a major pathogen of gorgonian sea fans in Southeast Asia (Fig. 1).