Reduced fenitrothion susceptibility in Haitian Anopheles albimanus is documented using time/mortality measurements from otherwise standard World Health Organization (WHO) bioassays. Survival beyond a time threshold in bioassays is shown to be highly correlated with elevated non-specific esterase levels. A shift in resistance incidence from less than 20 to over 60% in a six-month period is documented using both the bioassay and microassay procedures, showing the potential of microplate assay methods in early detection of resistance. Conventional 24-hour reading of WHO bioassay data failed to detect resistance until the level reached 60%. Resistance appeared to be focal, with an increase in intensity coinciding with a fenitrothion spray cycle for malaria control.