The objectives of this research were to evaluate the response to day length and the yield stability of environments of 4x families from 4x×2x crosses in potato. Twenty families from crosses between 4x female parents with different genetic backgrounds and 2x parents, which formed 2n pollen by first division restitution and were selected under short days, were evaluated in four locations in Peru (short days) and during two years at Rhinelander, USA (long days). Results from the USA indicate that the 4x progeny from 4x×2x crosses were very late maturing and poor yielding. Further, vine maturity, tuber size and tuber number are highly correlated with the percentage of Tuberosum germplasm in the parentage. Results from Peru indicate that most 4x×2x families had high tuber yield, stability for yield and high dry matter content. The lack of correlation between rank order of the parental value, based on progeny testing, of the male and female parents among short day locations and between short and long day environments indicates that selection of parents must be done in each location in which the final product, either a family or a clone, will be used. © 1991 Kluwer Academic Publishers.