In rasping aquatic feeders, like many tadpoles, it is challenging to measure both the food consumed and excreta produced and thereby calculate digestive efficiency. The goals of this study were to measure the apparent digestive efficiency of Northern Leopard Frog (Lithobates pipiens) tadpoles on an artificial laboratory diet and test the use of putative inert markers to calculate digestibility. The diet used was commercial ground rabbit chow (primary ingredient: alfalfa meal) suspended in agar and gelatin. If dietary ash (8.35% of diet dry mass) were a perfect inert marker, then its concentration in digesta, excreta, or both, should increase as organic matter is absorbed from the food, making the ratio of ash in food to ash in digesta or excreta an index of how much of the food mass is indigestible. Ash content in digesta dissected from the distalmost portion of the tadpoles’ intestine was about twice as concentrated as in food, indicating a 50% dry matter digestibility (DMD). This compared well with a DMD value based on our direct measurement of food intake and excreta production (50 ± 5%), which did not vary with Gosner stage (Stages 33–41 tested). Excreta collected from the bottom of the holding tank did not yield accurate estimates of DMD. We conclude that total ash of food and digesta can be used to measure DMD when L. pipiens is offered this diet.