Desirable genetic variation is required for selection progress and genetic gain in plant breeding populations. Genetic variation from exotic germplasm has been proposed to improve grain yields in temperate maize (Zea mays L.) breeding programs. Adaptation barriers should be overcome in exotic material prior to selection for trait improvement in temperate environments;Backcross introgression of elite exotic inbreds into temperate breeding material was initiated in 1995. Subtropical inbreds from CIMMYT heterotic group A and B were crossed to Reid Yellow Dent (RYD) and Lancaster Surecrop (LSC) materials, respectively. Objectives were to backcross with the adapted recurrent parent to ehance adaptation, select within semi-exotic material per se, and evaluate testcrosses of selected semi-exotic families outcrossed to inbreds heterotic to the recurrent parents. Recurrent parents per se and in testcrosses were used as checks to measure the effect of the exotic alleles in the backcross progenies;Semi-exotic backcross families (BC1F1), F1s, and recurrent parents were evaluated in Iowa during 1996. Adapted flowering in the semi-exotic crosses indicated the absence of major genes for lack of adaptation. High parent heterosis for grain yield was 36.7% for the RYD-exotic crosses, and 56.6% for the LSC-exotic crosses. Grain moisture, root lodging, and stalk lodging increased with increasing percentages of exotic germplasm;Semi-exotic BC1F1 testcrosses were evaluated in five Iowa environments during 1998, with average grain yields of 95.7 q ha-1 (153.1 bu ac-1). Selected testcrosses had grain yields similar to or greater (p ≤ 0.05) than their checks. Grain moisture of selected LSC-exotic and RYD-exotic testcrosses was similar to and greater, respectively, than the recurrent parent testcrosses, and resistance to root and stalk lodging was similar to the checks;Results support backcross introgression to incorporate alleles from exotic sources, to maintain the agronomic traits of the recurrent parent, and to maintain or enhance the combining ability of the recurrent parent heterotic pool. Inbred lines developed in CIMMYT's hybrid program have been improved for agronomic traits, for tolerance to inbreeding, and heterotic alignment. Pre-selected exotic germplasm represents a valuable resource for widening the genetic base of temperate maize.