After prolonged incubation, a small number of colonies of A. pullulans grew within zones of inhibition induced by copper in solid medium. There were no significant morphological differences between these colonies and control colonies of similar age. Yeast cells and chlamydospores exhibited similar responses to copper, germinating cells being particularly sensitive. X-ray microprobe analysis showed that the majority of copper taken up was located in the cell wall. The origin of the colonies did not appear to depend on any physiological or genetical adaptations.