We conducted serodiagnostic testing for dengue virus infection, murine typhus, scrub typhus and leptospirosis in Plasmodium falciparum-infected individuals in Thailand. Sera from 194 malaria patients with a median age of 24 years were tested. No antibody titers diagnostic of dengue virus infection were demonstrated, but 29 (15%) of patients had serological evidence of scrub typhus, 45 (23.2 %) patients had evidence of murine typhus, and 15 (7.7%) sera tested positive for leptospirosis. Our serological results suggested that duel infections are not uncommon in malaria that is acquired in Thailand. However, our results must be confirmed by prospective studies aimed at describing the causative organisms. Mixed infections would have multiple implications for clinicians, including unexpected clinical findings and apparent poor responses to antimalarial treatment in patients thought only to have malaria.