Malaria was studied in the province of Takeo, Cambodia. In the hyperendemic region of Kirivong district, 49 (46.0%) of 105 suspected patients were found to be infected, 98% of them with Plasmodium falciparum and 2.0% with P. vivax. The highest prevalence (85.7%) was recorded in the group of 15-20-year-old patients. A total of 296 patients were examined in the hospital of Takeo during one year (1983-1984) and 77 (26.0%) of them were positive. P. falciparum infection was found in 76.6% and P. vivax in 23.4% of cases. The highest prevalence (42.3%) was also in the age-group of 15-20 years. The "7-day test" was used in vivo in 15 patients in order to detect the sensitivity of P. falciparum to Fansidar. The asexual forms of parasites disappeared within 4 days, while the gametocytes survived in two patients until 7th and 8th day, respectively. The observations could not be terminated, since the two patients will fully left the hospital.