Genotype × environment interactions in sweet potato were examined in the highlands of Papua New Guinea by evaluating nine contrasting sweet potato varieties, including newly introduced and established varieties, at six sites covering a range of climatic and soil conditions. Statistically significant genotype × environment interactions occurred, but the pattern of these interactions was related primarily to the variation in varietal response to differences in yield potential between sites, and in varietal ability to yield at sites with poor yield potential. Newly introduced varieties were more responsive than established varieties to differences in yield potential between sites. Established varieties were able to give some yield at poor sites, a characteristic shared by only some of the introduced varieties. Genotype × environment effects were also examined for variety susceptibility to leaf scab. Although significant interactions occurred, these results were not related to effects on yield.