A severe infestation of adult host-seeking black flies (Diptera: Simuliidae) occurred in west central Florida during 1998. Collections from stationary suction traps in Pasco County revealed the presence of large numbers of Simulium slossonae Dyar and Shannon. This species peaked in traps during March (avg >40 per trap) with a lesser secondary peak in October (avg ≈5 per trap). Moreover, during March, some suction traps had collected as many as 2,000 black flies for the month. It was believed that the spring outbreak of S. slossonae was the result of above average precipitation associated with an El Niño event. Precipitation produced by this weather system during the winter of 1997/1998 provided a continuous source of rain-swollen ditches, streams, and creeks for rapid larval and adult production the following spring. Conversely, 1999 resulted in rainfall deficits of 1.5 cm to nearly 7.0 cm below normal. During that year, adult black fly populations were almost nonexistent (≤3 black flies collected per trap month) compared with collections obtained the previous year.