Aspergillus fumigatus is a pathogenic fungus capable of causing both allergic lung disease and invasive aspergillosis, a serious, life-threatening condition in neutropenic patients. Aspergilli express an array of mycotoxins and enzymes which may facilitate fungal colonisation of host tissue. In this study we investigated the possibility of using the insect, Galleria mellonella, for in vivo pathogenicity testing of Aspergillus species. Four clinical isolates of Aspergillus fumigatus and a single strain of Aspergillus niger were characterised for catalase and elastase activity and for the production of gliotoxin. Gliotoxin is an immunosuppressive agent previously implicated in assisting tissue penetration. Results illustrated a strain dependent difference in elastase activity but no significant difference in catalase activity. Gliotoxin production was detected in vitro and in vivo by Reversed Phase-High Performance Liquid Chromatography, with highest amounts being produced by A. fumigatus ATCC 26933 (350 ng/mg hyphae). Survival probability plots (Kaplan-Meier) of experimental groups infected with Aspergillus conidia indicate that G. mellonella is more susceptible to fungal infection by A. fumigatus ATCC 26933, implicating a critical role for gliotoxin production rather than growth rate or enzymatic activity in the virulence of A. fumigatus in this model.