Temperate bats make extensive use of caves and mines as nursery roosts, swarming sites and hibernacula. For a variety of reasons, the entrances to many sites have been modified in the past to restrict human access. Early barrier design often gave little regard to bats, leading to massive population declines in many nursery and hibernation sites. Free access to bats has become an increasingly important design feature, as the damaging effects of early gates were recognised. However, given the large number of gates that have been constructed, relatively few studies have looked at either the short or long-term effects of gates on bat behaviour and population sizes. Even fewer studies have examined specifically the effects of different gate designs. We have looked at the immediate effects of gates on the behaviour of swarming bats as they entered a natural cave. Three gates were tested, all with vertical grille spacings of 750 mm, but with horizontal spacings of 150, 130 or 100 mm. The gate with 150 mm spacings had no significant effect on the behaviour of the bats (predominantly Myotis nattereri). Gates with both 130 mm and 100 mm spacing caused a significant and substantial increase in the number of bats aborting their first and often subsequent attempts to enter the cave. The consequences to swarming behaviour and long-term use of the site by bats are unknown, but we suggest that following the precautionary principle, the minimum spacing between horizontal bars in gates should be 150 mm.