The total diffusible content of ammonia in Oniscus asellus was measured as 1.20 mg % body fluid. Daily excretions of ammonia exceeded the total diffusible content in measurements on specimens taken directly from nature. The content of uric acid in a group of 35 specimens was 11.3 mg % body weight and 93% of this material was in the body wall. Urea was not measurable under conditions that would have allowed detection of at least 0.26 mg % body fluid. Only arginase of the enzymes in the ornithine-urea cycle was measurable. Labeled urea was not detectable in aqueous extracts under conditions where approximately 50,300 CPM of labeled carbon was incorporated into metabolites. Labeled arginine was not detectable under conditions where 9,100 CPM of KHC14O3 were incorporated into the soluble protein fraction and more than 3,340,000 CPM were incorporated into particulate protein and non-protein fractions of the organism. All of the enzymes of uricolysis to ammonia were present, uricase and urease being rate-limiting. Uricase was measurable as myeloperoxidase. Although high rates of glutamate-oxalacetate aminotransferase and glutamate-pyruvate aminotransferase were readily measured, glutamic dehydrogenase activity proceeded slowly, suggesting synthesis of needed amino acids as the key function of the transaminase activities. Amino acid oxidase activities, glutaminase, and asparaginase activities were low, too, whereas peroxidatic deaminase activities proceeded at rates sufficiently fast to account for the levels of volatile ammonia emitted in vivo. Although molting may serve for excretion of end-products of purine catabolism, no central organ homologous to kidney or liver appears to be present with respect to detoxication or excretion of ammonia. Inasmuch as O. asellus is not exposed to osmotic stresses, does not ordinarily face problems of dehydration, and tolerates levels of ammonia generally ascribed as toxic to other organisms, it is proposed that retention of ammonotelism probably offers thermodynamic advantages to the organism. The evolution of a system wherein ammonia is excreted as a gas may have provided the organism with an adaptive mechanism for colonizing land.