The objective of the present study was to investigate activity time of the molossid bats Nyctinomops laticaudatus and Molossus molossus at the same locality, and to test whether activity of one species affects activity of the other. During January 2007-May 2009, we sampled for 15 nights over a stream in southeastern Brazil. Total sampling effort was 166 h and 693 m of nets. Time of capture was transformed into minutes in relation to sunset. First captures of M. molossus occurred just after sunset except for one individual that was captured before sunset. Total activity time of M. molossus ranged from 27 min before sunset to 743 min after sunset. Total activity time of N. laticaudatus ranged from 42 to 675 min after sunset and activity differed significantly between species. Activity of M. molossus was related to time of sunset, as expected based on behavior of other insectivorous species, whereas activity of N. laticaudatus seemed independent of sunset.