Among the methods to evaluate field diets of insect predators is frass analysis. The potential of this infrequently used method was explored for determining prey consumption by adult ladybird beetles (Coleoptera: Coccinellidae) in alfalfa, Medicago sativa L., fields. A laboratory experiment revealed that at 20°C, within 48 h after consumption of prey [either pea aphids, Acyrthosiphon pisum (Harris), or larvae of the alfalfa weevil, Hypera postica (Gyllenhal)], almost all prey fragments had been voided by adults of five ladybird beetle species commonly occurring in Utah alfalfa fields. Among the five ladybird beetle species, aphid and weevil fragments were detected in frass of 80–95 and 29–73% of adults, respectively. A second laboratory experiment with adults of the most common of these predators (the introduced Coccinella septempunctata L.) more fully identified and enumerated diagnostic fragments for specific types of prey (aphids, weevil larvae, and conspecific larvae) as they occurred in frass. Frass pellets from the consumption of alfalfa weevil and conspecific larvae most often contained diagnostic cuticle and setae, and less often mandibles and head capsules. Frass from the consumption of pea aphids most often contained antennae and legs, and less often mouthparts, eyes, and tarsal claws. These laboratory results provide a foundation for using and interpreting the results of frass analysis as a technique to assess field diets of aphidophagous ladybird beetles in alfalfa.