Current techniques used in genetic transformation can result in variation of numerous traits in addition to the transformed trait. Backcrossing to the standard genotype can eliminate this variation, but because of the heterozygous nature of potatoes (Solanum tuberosum L), backcrossing is not effective. Therefore, the chances of obtaining altered performance in transformed potato are high. 'Superior' potato plants were recently genetically modified to resist attack and damage by the Colorado potato beetle [Leptinotarsa decemlineata (Say)]. The transformed clone, 'NewLeaf Superior' ('NewLeaf'), has been shown in previous field trials to be more vigorous than the standard clone. The objective of this 2-year study was to evaluate the performance of 'NewLeaf' relative to that of the standard clone at various fertilizer nitrogen (N) levels. The two clones were randomly assigned as subplots to main plots consisting of four N levels (28, 112, 224, or 336 kg·ha-1). Based on regression analysis, total yield was higher for 'NewLeaf' than for 'Superior' at N rates below 92 kg·ha-1 in 1997. At higher rates, however, 'Superior' had higher yields than the transgenic clone. In 1998, the clone x N rate interaction was significant, but there was no consistent trend to the response of the two clones to N application. At the 112 kg·ha-1 N rate, total yield was higher for 'NewLeaf' than for 'Superior', but yields were similar for the two clones at other N rates investigated. Nitrogen and biomass accumulation in vines increased more for 'NewLeaf' than for 'Superior' as N rate was increased from 28 to 336 kg·ha-1. At equivalent N rates, these traits were higher for the transformed than for the standard clone within the range of N rates investigated. However, harvest index at equivalent N rates was higher for the standard clone than for 'NewLeaf'. 'Superior' and 'NewLeaf' produced similar tuber dry weight yields per unit of N supplied and per unit of N absorbed by the plant. Nitrogen uptake efficiency (NUE) was 16% higher for 'NewLeaf' than for the standard clone at the low N rate (112 kg·ha-1), whereas at higher N rates NUE was either lower for 'NewLeaf' or similar for the two clones. This observation, together with the finding that yield for 'NewLeaf' was maximized at lower N levels than the standard clone, suggests that 'NewLeaf' may require lower N input than the standard clone. Results from the study indicate that the greater efficiency of 'NewLeaf' at lower N levels was associated with acquisition of N from the soil rather than utilization of absorbed N in metabolism.