An experiment was conducted during the 1998 and 1999 cropping seasons at six different locations in eastern Ethiopia to study the phenotypic stability of nine medium-maturing elite potato (Solanum tuberosum L.) genotypes and to understand the nature and magnitude of genotype by environment (G x E) interaction in potato. At all locations, the experimental plots were arranged in a randomized complete block design with three replications. Significant differences among genotypes were observed and the G x E interaction was also significant. Both linear and non-linear components of G x E interactions were found to be important with a predominance of the former. The CIP-389668-4 clone exhibited above average responsiveness along with stability, indicating that it had specific adaptability for favourable environments. The next highest-yielding clone was CIP-388764-26 with stable average responsiveness, indicating that it had general adaptability. The third highest-yielding genotype (CIP-387412-2) was unstable. Similarly, Chiro, the widely grown potato genotype (control) was found to be unstable. Clone CIP-388764-26 is recommended for regional release.