Physiological responses of wildlife species to zoo visitors should be studied to better understand how wildlife perceive human encounters. We conducted an experimental test of the effect of changes in zoo visitor encounter experiences on the glucocorticoid (GC) response of koalas (Phascolarctos cinereus) in a Sydney zoo. Koalas were housed in a multiple-bay enclosure (two to three koalas per bay) for photography sessions with zoo visitors (no touching of koalas permitted by visitors). Following a one-week no-photography baseline period, photography sessions were rotated between three enclosure bays for four weeks (Intensive photography), then between five enclosure bays for an additional four weeks (Standard photography). A sixth enclosure bay was never included in the photography sessions (control bay); koalas in this bay showed no significant change in fecal cortisol metabolites (FCMs) during the course of the study. In the five experimental bays differences were seen between male and female koalas. Males had higher mean FCMs than females, and individual FCM traces showed that two male koalas that were related and of similar age responded strongly to the experimental manipulation. These two males showed a peak in FCMs at the beginning of the Intensive photography period, then a decline when photography sessions returned to the Standard protocol. No systematic pattern in response to photography sessions was observed in females. Our results demonstrate successful application of a non-invasive endocrinology tool for assessing the stress biology and welfare of captive zoo wildlife.