AbstractIt has been shown that increased grain yield of modern cereal cultivars is associated with impaired phenotypic stability. This study, consisting of 19 Finnish and Nordic oat (Avena saTiva L.) genotypes, confirmed this and, in addition, indicated that the most unstable genotypes were characterized by their poor partitioning ability. Correlations between varietal regression coefficients of grain yield and 16 morpho-physiological traits (representing maturity class and plant architecture) indicated that long strawed genotypes with abundant above-ground biomass associated with low HI, were unstable in yield performance. Furthermore, especially late maturing genotypes with a long grain-filling period and high single grain weight tended to suffer most from unfavorable post-anthesis growing conditions. Thus, breeding for genotypes with increased phytomass production, associated with efficient economic sinks, i.e. panicles with high numbers of grains, does not necessarily result in weakened phenotypic stability. The importance of high partitioning ability and sink capacity in producing genotypes with stable yield performance is discussed.