We quantified factors contributing to failed or successful prey captures by Pipa pipa, a permanently aquatic, tongueless frog widely distributed in Amazonian South America. Pipa catches fish by entraining and ingesting large volumes of water and by limiting fish escape with its fingers. Based on analysis of high-speed video (250 and 500 fps), feeding attempts appeared superficially stereotyped, but many features were modulated and slower than in other suction feeders. For both successful and unsuccessful capture attempts, the entire frog might or might not move and fingers might or might not contact the prey. Mouth opening generated initial suction but continued movement of entrained water depended on actively enlarging the volume of the anterior trunk by depressing the ventral pectoral girdle. Although captured fish varied in size and position relative to the frog, both fish size and the distance of the fish from the frog's mouth at the initiation of mouth opening were significantly greater for unsuccessful attempts. Our data suggest that capture success depends partly on sensory evaluation of prey size and distance that initiate capture movements and partly on the independent probabilities of rapidly moving parts in two different organisms favoring the predator or its prey.