Landing counts made in a residential area in San Juan, Puerto Rico during the 5-year period, 1980-84 indicated that malathion ULV applied by LECO Fog Generators was not effective against Aedes aegypti. Of 25 applications only 6 resulted in a decrease on the first day after treatment and only one remained much less than the pretreatment count on the second and third days after treatment. Daily, the number of mosquitoes landing for a 5-minute period at each of 2 stations was recorded. An ideal year created by averaging the monthly data suggested that the seasonal variation of mosquitoes was inverse to that of reported dengue incidence.