For a study of the toxicology of P against large nymphs of the American and German cockroaches (Peri-planeta americana and Blattella germanica) a commercial, 2% P paste was used as the source of P. In this product P is finely dispersed and suspended in a viscous medium. By quantitative feeding of P paste diluted with syrup, the M.L.D. of P for the American cockroach was found to be 0.02 mg. per g., showing that it is a highly toxic stomach poison for this species. P was much more toxic to the American cockroach than was Na arsenite or NaF when the latter were fed in the same viscous medium and P was imbibed more rapidly and with less regurgitation than were the other 2 poisons. Na arsenite was more toxic in water than in syrup, but even in water it was less toxic than P. The German cockroach was less susceptible to P in paste diluted with syrup than was the American cockroach, the M.L.D. for the former being 0.13 mg. per g. as compared with 0.02 mg. per g. for the latter. The larger M.L.D. for the German cockroach cannot alone account for the failure of P paste to control this species in practice. It is probable that the German cockroach does not imbibe paste as freely as does the American cockroach. Studies were made of the effect of P in paste when administered to the American cockroach by injection, by contact and by exposure to fumes. P in paste diluted for injection with a physiological salt soln. appeared to be somewhat less toxic than when administered by mouth in the same medium. P in paste killed cockroaches when painted on the body, the mouth-parts being shielded from contact with the paste. In a small closed space, cockroaches were killed within 24 hrs. by P paste with which they were not in contact.