Aspergillosis was diagnosed as the cause of mortality during an epizootic in which 32 of 140 captive herring gulls (Larus argentatus) died. The cause of disease was attributed to the individual or cumulative stresses of capture, transportation, confinement, and a nutritional deficiency allowing subclinical levels of Aspergillus fumigatus fungi present in the gulls at the time of capture to produce overt disease.