Between 1978-1979 an epidemic of yellow fever occurred in Trinidad in two phases. Eight cases occurred in December 1978-March 1979 and 10 cases occured in August-September 1979. Seven died, giving a case fatality rate of 39 per cent . All cases were male aged 16-58 years (median 22.5). Seventeen had been exposed in forests where the virus was active and none had records of being vaccinated. The high level of clinical and virological surveillance that existed made it likely that all or nearly all the cases were ascertained. Intense immunization, health education and vector control efforts also mitigated against a larger epidemic.