Technical Abstract: Stability of parameters describing growth of cool-season annual clovers is important because of the great interest in utilization as winter cover crops, forage and sources of nitrogen through N2-fixation. Accordingly, the objective of this study was to quantify the key parameter for biomass production, radiation use efficiency (RUE), for arrowleaf (Trifolium vesiculosum Savi.), crimson (Trifolium incarnatum L.), rose (Trifolium hirtum All.), and subterranean (Trifolium subterraneum L.) clovers. Data on fraction of light intercepted and biomass, sampled biweekly over two years at Overton, TX were used to calculate the RUE. Radiation use efficiency was calculated for biomass as a function of cumulative intercepted photosynthetically active radiation (IPAR) for above-ground biomass and for total biomass including roots. Similarly, RUE was calculated for regrowth following one or two cuttings. The mean radiation use efficiency (RUE) of above-ground biomass original growth were 2.92 g MJ**-1 for rose, 2.52 for arrowleaf, and 1.94 for crimson, and 1.86 for subterranean. For total biomass including roots, original growth RUE values were 3.01, 2.59, 2.00, and 1.98, respectively. Compared to original growth above-ground RUE values, one cutting regrowth RUE averaged 92 percent whereas two cutting regrowth RUE averaged 119 percent. These values of RUE will aid modelers in simulating these important nitrogen fixing species.