Successful transmission of infections of Litomosoides carinii, a filarial parasite of the cotton rat Sigmodon hispidus texianus, has been accomplished in all of three trials made by housing together infected and uninfected rats in a moist environment into which freshly gathered grass nests of cotton rats had been introduced. Altogether 15 out of 16 cotton rats became infected as well as the only white rat tried, while 24 control cotton rats and 4 control white rats remained uninfected. The only biting arthropods found in the experimental units were fleas, Rhopalopsyllus gwyni, and two species of mites, viz. a new species of Atricholaelaps and Liponyssus bacoti. On the grounds of abundance both in the units and on wild-caught rats either the fleas or the former species of mites seemed indicated as the vector, but 100 and 500 dissections respectively revealed no developing larvae. L. bacoti was rare both on wild rats and in the units and too few were dissected for conclusions, but this species seems to be incriminated by the elimination of other possibilities.