The narrowness of the temperate maize (Zea mays L.) germplasm base has long been recognized, and there are many available, elite tropical lines that might be used to profitably broaden it. However, there are few comparative yield-trial data by which to choose which line(s) might be most useful. As the investment required for using a tropical line in a temperate breeding program is large, line-choice is critical. Here we report the results of testing a group of potentially useful tropical lines in topcrosses grown in North Carolina. Results for 50%-exotic topcrosses and for 25%-exotic topcrosses are compared, and the 50%-exotic topcrosses with a broad-based tester (here, LH132.LH51) appear to be most efficient for initial screening. In addition, virtually all crosses suggested that any superior tropical line could be used equally well with either Stiff Stalk or non Stiff Stalk germplasm. Of the 22 lines tested, CML258 and Tzi9 appear to be the most promising, if yield improvement is a major criterion. None of the lines appeared to have serious lodging, maturity, or moisture problems in either 25% or 50%-tropical crosses.