Blood from most of the 250 residents of a nonmiaratorv farming village in south-eastern Thailand was visually examined for malaria parasites monthly for 2 vears. Nearlv 97% of the oonulation had at least one (median=5) patent Plan&d&m falciparum infection per year; 72% had one due to P. vivax (median=l). This contrasted with a slide positivity rate of 17% calculated from 12 months of passive case detection before the study began. Children l-9 years old had the highest mean monthly prevalence (51%) and highest geometric mean den&v (lo/500 white blood cells) Gf P. falciparum. Fewer ihan half the expected number of mixed infections were found but these were more common at high densities of P. falcifxzrum. Individuals over 19 vears old comnrised 52% of the novulation but accounted for only i8% of P. vivaw a&i 32% of P. falciparum gametocytaemias. Fever rates were marainallv h&her in those below 10 vears old (8%) but oc'curred wyth equal frequency in ihose with patent infections or negative. The spieen rate (89% staee 1) was 24% in those under 15 vears old and 7% in &se older. No malaria mortality was seen.