Approximately 50% of matings in the frog Crinia georgiana involve two or more males. We report reduced fertilization success as a major cost of mating with multiple males. For single-male matings, fertilization success was consistently high averaging 96%. Only 68% of eggs were fertilized when females were amplexed by two males and this dropped to 64% when females were amplexed by three to five males. Multiple regression analysis revealed the reduction in fertilization success was significantly related to the number of amplectant males but not to clutch size or three measures of water quality (depth, temperature and oxygen concentration) at the site of oviposition. The most likely cause of reduced fertilization success is struggles amongst males which interfere with effective sperm transfer.