Opportunistic pulmonary infections are a major cause of post-transplant morbidity and mortality. Among these infections, Aspergillus is a common cause of fatal pneumonia. Owing to the precarious clinical condition of many patients who acquire invasive mold infections, clinicians often treat them on the basis of radiographic findings, such as the halo sign. However, in patients who do not respond to treatment or who have uncommon presentations, bronchoscopy or lung biopsy looking for other pathogens should be considered. This study describes two cases in which the radiographic halo signs characteristic of Aspergillus were in fact due to Legionella jordanis, a pathogen that has been culture proven only in two patients previously (both of whom had underlying lung pathology) and diagnosed by serologic evidence in several other patients. In immunocompromised patients, Legionella can present as a cavitary lesion. Thus, presumptive treatment for this organism should be considered in post-transplant patients who do not have a classic presentation for invasive fungal infection and/or who fail to respond to conventional treatment. These cases illustrate the importance of obtaining tissue cultures to differentiate among the wide variety of pathogens present in this patient population.