The metabolic fate of oxamyl was examined in mice and twospotted spider mites,Tetranychus urticae Koch. Mice treated intraperitoneally with oxamyl-14C eliminated 96.4% of the dose in the urine (88.7%) and feces (7.7%) by 96 hr; 75.7% of the dose was eliminated during the initial 6 hr. Organosoluble urinary radioactive material ranged from 24.6% at 6 hr to 6.5% at 72 hr and consisted of oxamyl, methylN-hydroxy-N′,N′-dimethyl-1-thiooxamimidate (DMTO), methylN-hy-droxy-N′-methyl-1-thiooxamimidate (MTO),N,N-dimethyl-1-cyanoforma-mide (DMCF), N,N-dimethyloxamic acid (DMOA),N-methyloxamic acid (MOA), methylN′-methyl-N-[(methylcarbamoyl)oxy]-1-thiooxamimidate (DMO), and several unknowns. Although some of the water-soluble urinary radioactive material was cleaved withβ-glucuronidase and aryl sulfatase, most of it was resistant to cleavage by enzymes and acid (9% HCl). Tissue levels of radiocarbon were low. Oxamyl metabolites formedin vitro by mouse liver subcellular fractions included DMCF, DMOA, MOA, DMTO, DMO, and several unknowns. Twospotted spider mites absorbed and metabolized oxamyl. The total internal radioactive material increased from 6.1% at 1 hr to 44.5% at 24 hr. Radioactive compounds isolated from two-spotted spider mites in addition to oxamyl included DMCF, DMOA, MOA, DMTO, DMO, and several unknowns.