A direct comparison was made of radiotracer and gravimetric measurements of ingestion and assimilation rates, and of assimilation fraction, in Armadillidium vulgare under laboratory and field conditions. The elimination 85Sr was also measured in the laboratory and field. Radiostrontium was excreted more rapidly in the field. The radiotracer and gravimetric measurements gave similar estimates of ingestion and assimilation rates, and of assimilation fraction. Assimilation fraction varied, ranging from 0.06 to 0.13 in the laboratory to 0.53–0.75 in the field. Assimilation fraction appears to be inversely related to rate of ingestion, resulting in a relatively constant assimilation rate over a wide range of ingestion rates. Conversion of radiotracer and gravimetric estimates to caloric estimates, using bomb calorimetry, revealed a greater caloric value of assimilated material than of the food ingested.