Plants of seven perennial Trifolium species were interspecifically pollinated. The female flowers opened at 40 °C, and the male flowers opened in the greenhouse or field without a temperature treatment. T. pratense was pollinated with T. ambiguum, T. hirtum, T. ochroleucum, T. rubens, T. sarosiense, and T. medium. T. medium was pollinated with T. pratense. Seeds were obtained from all pollinations ; however, 86 of the progeny were morphologically and cytologically similar to their female parent, and probably resulted from self-fertilization. Many seeds of possible hybrid origin were small and/or did not germinate. Some seeds produced weak or albino seedlings which died at an early age. Excised pistils of T. medium from flowers that opened at 40 °C were more compatible with pollen of T. pratense than pistils of T. medium that developed in the field, indicating that the elevated temperature caused greater hybrid seed production than was observed in our earlier studies. The high temperature technique may prove useful in obtaining hybrids in Trifolium.