Marinduque has never been considered endemic for filariasis. The disease was suspected when a high incidence of scrotal enlargement was noted in three villages. On April 25-26, 1991, a field survey was conducted in these villages. The objectives were: 1) to confirm the existence of filariasis, 2) to determine its age and sex distribution, 3) to gain insights on its prevalence and on the periodicity of the parasite. Daytime and nocturnal blood films were collected from both sexes aged 5 years and above. They were examined for microfilaria. Dried up smears were examined after staining. A total of 297 individuals comprising 23% of the area's population were studied. Microfilaremia was demonstrated in 47 individuals or 16% of those examined. Among the positive cases, 32 or 68% were males. Individuals 15-49 years old accounted for 28 or 60% of all the cases discovered. The prevalence rate for the study area was 37/1000 population. Out of 146 daytime smears, 18 or 12% demonstrated microfilariae. Of the 224 nocturnal smears, 37 or 17% were positive. Parasitologists at the Department of Health identified the parasite to be Wuchereria bancrofti. The study established the presence of filariasis in Marinduque. Males and young adults are predominantly afflicted. A non-periodic strain of parasite is possibly involved. Further investigations of the parasite and the vectors are recommended.