Book excerpt: Abstract. Twenty-one progenies of smooth bromegrass (Bromus inermis Leyss.) from a 7x7 half diallel cross with their parents were evaluated at four locations in Alberta and in two years for the genetic variation and stability in expression of their yield potentials along with leaf and tiller characteristics. Years, locations and their interactions were highly significant in the combined analysis of variance, and each of these effects was therefore treated as an individual environment in subsequent analysis. The partition of the genotypic variance into general combining ability (GCA) and specific combining ability (SCA) showed that for all characters except spring, fall and annual yield, GCA was more important than SCA, indicating the importance of additive genetic effect. Although the genotype x environment (GE) interaction were highly significant, variation accounted for by combining ability effects was generally higher than the interaction effects of GCA and SCA with environments. The joint regression analysis further partitioned the GE interaction into heterogeneity among regressions and its residual. A significant part of the interaction was ascribed to the heterogeneity among regression lines for all characters and therefore was predictable. However, the residual components were significant in most cases. Hence there was also the presence of some unpredictable and unaccountable variation. Regression coefficient and deviation from regression line for each genotype were the two stability parameters to be considered together with mean performance in the evaluation of each genotype. Five high yielding genotypes, namely 12, 13, 16, 25 and 36, had general adaptability, while genotypes 23 and 26 were specially suited to poor environment. Combining ability analysis revealed that GCA and SCA were both important in the expression of mean yields (spring, fall and annual). Inheritance of linear regression was controlled predominantly by GCA whereas both GCA and SCA were equally important in the expression of deviation. Genotype 1 was the most desirable parent, as it transmitted to its progeny high yield potential and average linear response. The other good combiner for yield was the genotype 6 but it transmitted below average linear response. SCA analysis revealed that genotypes 12, 13 and 16 combined both high yield and average stability. Genotype 34 was found superior in favourable environments, but its non-linear fluctuations were rather high. Phenotypic and genotypic correlation coefficients indicated that all the yield types, including spring, fall, annual yields, and yield per area, were all significantly correlated. Very few morphological characters seemed to associate with yields, among these were tiller density and plant height. Beside these two characters, other characters such as leaf area, leaf, stem and tiller dry weight were also correlated with yield per area but not with other yield types. Both direct and transformed scales were used throughout this study. Tests of significance among means were generally similar on both scales, but for heterogeneity among regressions, the tests produced different results. The results indicated that a more complicated approach, such as recurrent selections involving multi-location and multi-year testing, seems necessary in breeding for high yielding bromegrass cultivars in Alberta.