A mode of infection with pinworms is descr., whereby a primary infection may be continued by chronic "retrofection," meaning the peranal ascent of the larvae which hatched on the skin round the anus. After peranal application of about 50 larvae to 3 volunteers the infection became manifest once on the 43d and twice on the 76th days after infection, attacks lasting 13-26 days. In 2 chronic cases infections died out spontaneously after 18 and 16 mos.; in 2 other cases they were still in progress after 20 and 10 mos, respectively. This characteristic picture of oxyuriasis, type R, can be differentiated easily from the other 3 types, i.e., type D, digital; type C, contact; and type A, air-borne. The new type R clears up hitherto unexplained cases of oxyuriasis.