1. Observations on filariasis made during medical travels in the Malay Peninsula are described. 2. The tentative diagnosis of endemic filariasis was made when cases typical of filarial elephantiasis were found in members of the indigenous population who have never resided in a previously known filariasis area, and was confirmed by finding microfilariae of Wuchereria malayi in bloods from that population. 3. Endemic filariasis has previously been reported associated with jungle swamp along the lower reaches of some of the larger rivers, and in certain coastal ricefield areas. It is reported in this paper in undeveloped inland areas of Perak, Pahang and Selangor, far distant from the previously described foci. This data has been summarized in maps and an Appendix. 4. In most inland areas where a search has been made, it has been possible to find evidence of endemic filariasis and sometimes the parasite rate has been over 50%. 5. The geographical distribution of the disease has not yet been defined, but is certainly more extensive than that described in this paper. 6. Infection probably takes place at an altitude of 1,500 feet in mountain valleys in Malaya.