||Hybrid-oriented source germplasm with established heterotic pattern is essential for initiating hybrid development. The objective of this study to identify and from heterotic groups of maize (Zea mays L.) with subtropical adaptation. Eighty-eight inbred maize lines derived from six CIMMYT subtropical maize populations and pools were crossed to four tester lines, one each from Pool 32 and Populations 34, 42 and 44. The 352 line x tester hybrid combinations were divided into four sets, each set comprising of crossed of 22 lines with the four testers, and evaluated during two seasons at Tlaltizapan, Mexico during 1989 and 1990. Mean grain yields for the four sets ranged from 9.0 t/ha (set 1) to 8.2 t/ha (set 4). General combining ability (GCA) and specific combining ability (SCA) for grain yield were calculated by line x testers analysis. Of the 88 lines tested, 14 from Population 44, 11 from Population 42, nine from Pool 32 and one from Population 34 had positive GCA effects for grain yield. Among testers, Tester 2 (Pop. 44) showed positive GCA effects for yield, and Tester 1 (Pool 32) had negative GCA effects for yield. Significant differences were observed for SCA effects for yield in the different line x tester crosses. Several combinations were identified having yields of 10 t/ha or more and possessing high SCA effects. Interpopulation crosses generally outyielded and had greater SCA effects as compared with intrapopulation crosses. Superior intrapopulation combinations, however, were observed among crosses involving lines from Populations 44. Two heterotic groups STHG "B" are being formed from these materials using the testcross data. Several of the lines included in this study were announced as CIMMYT maize lines (CML) during 1991 and are made available to our cooperators worldwide. Twelve of the 20 top-yielding single crosses (fice from each set) were between CML lines. The material and information generated from this study may be useful for future hybrid development work at CIMMYT and in other public and private breeding programs, particularly in the developing world.