A 10 parent diallel was formed to determine combining ability and heterotic patterns among CIMMYT'S tropically adapted, early and intermediate maturity maize (Zea mays L.) gene pools and populations. The parents and their crosses were avaluated for yeild (t/ha), plant and ear height (cm), and days to silk at five locations in México, and one each in Colombia, Ecuador, India and Thailand. The test for average heterosis, parents vs. crosses, was significant for graind yield, and plant and ear height in the combined analysis of variance. General combining ability (GCA) was significant for all traits. Specific combining anility (SCA) was significant anly for ear height. Although yield heterosis over the better parent was low in most crosses, moderate levels were observed in the Population 49 x Population 26 cross (9.6%). Population 26 also combined over the better parent. Pool 22 had the highest GCA effect for yield (0.37 t/ha), and was a parent in three of the five top yielding crosses. High yielding combinations included Pool 22 with Pool 20 (6.33 t/ha), Population 23 (6.24 t/ha), and Population 26 (6.23 t/ha). However, maximun heterosis over the better parent was only 3.2% in these crosses. The only cross wiht a significant positive SCA effect for yield was Population 23 x Pool 20 yielding 6.13 t/ha with 6.7% heterosis. Heterosis for plant and ear height, and days to silk were generally low. Based on our study, the best choices for initiating hybrid work among white grain materials are Population 23 and Pool 20, and among yellow grain materials Populations 26, Pool 21, and Pool 22.