Unmated frogs and mated pairs from a population of individually marked Hyla labialis were captured, measured and released over four consecutive years in a highland valley of the Colombian Andes. Across eight prolonged breeding seasons, three times more males than females were captured. On average, females were significantly larger than males. Although the largest reproductive males were larger than the smallest reproductive females, amplexing males were always smaller than the females they clasped. Larger males had no mating advantage, because the body size distribution of males was the same for mated and unmated males, and the average snout-vent length of mated males was similar to that of unmated males. There was no indication of larger males taking over clasped females during male-male competition, because there was no size difference between males clasping gravid, spawning and spent females. Significantly size-assortative matings occurred on days when male availability per female was moderately high, but not when it was low, suggesting that female mate choice is relatively more important than male-male competition in shaping mating pattern.