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Development of inbred lines is a routine activity in hybrid maize breeding program.Understanding the relative importance of general (GCA) and specific (SCA) combining ability effects fordifferent traits of newly developed maize lines is of paramount importance to design future breedingstrategies for the development of hybrid. There are several types of mating designs available to study thecombining abilities of maize inbred lines, of which line x tester mating scheme is one used to determinethe combining abilities and also to categorize the lines into heterotic groups. The objectives of the studywere, therefore, to estimate GCA and SCA of newly developed highland maize lines using line x testermating design, to identify best promising hybrid crosses with desirable traits for further breeding andcultivar development, and to identify the heterotic groups of the lines involved in the crosses. Twelvelines and four testers were involved in the study. A total of 48 testcrosses along with two checks weremade available for evaluation across locations. An alpha lattice design (10x5) with two replications at eachlocation was used. Two lines (L6 and L5) showed highly significant and positive GCA effects for grainyield. Besides, L6 also revealed positive GCA effects for the other yield related traits such as number ofears per plant (NEPP), number of rows per ear (NRPE), number of kernels per row (NKPR) and eardiameter (ED). On the other hand, crosses L7XT2, L8xT4, and L9xT1 had positive and significant SCAeffects for grain yield. The lines and crosses with positive and significant GCA and SCA effects,respectively; could be selected for use in highland maize hybrid or/and synthetic cultivar development.The testers clearly categorized L3 and L6 into their respective heterotic groups in a similar pattern unlikethe other lines which requires the application of molecular markers for further verification.