Hourly measurements of meteorological conditions and black fly activity were made during daylight periods of 18 days from June 20 through Oct. 18,1975. Analysis of data indicated that the behavior of ♀ simuliids attracted to host-simulation traps is affected by cloud types, light intensity, and time of day. Combinations of such variables as relative humidity, barometric pressure, percentage of cloudcover, and the hourly change in light intensity accounted for less than 25% of simuliid activity at individual trapping sites. Simuliid activity therefore appeared to be more dependent upon unmeasured changes in microhabitat. Models for diurnal activity showed consistent patterns of behavior. Activity was greater in the presence of cumuliform clouds than in the presence of stratiform clouds.SimuliumslossonaeDyar and Shannon, a primary vector ofLeucocytozoon smithiLaveran and Lucet, 1905, in turkeys, was the predominant species at one trap.