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We experimentally tested the hypothesis that three-dimensional structural clutter is a major factor affecting habitat choice by insectivorous bats. To do this we artificially increased clutter at two sites used by bats without affecting prey availability. Our measure of available insect prey did not differ between treatment and control sites, indicating that only the spatial complexity of the habitat type was manipulated. Artificial clutter negatively affected foraging activity by small bats (Myotis spp.), supporting the hypothesis that physical clutter does affect foraging by bats. In direct contrast to our expectation, large bats (Eptesicus fuscus, Lasiurus cinereus, Lasionycteris noctivagans) were apparently unaffected by the clutter treatment. We suggest that this may be an artifact of our experimental design, because our ultrasonic detectors probably recorded large bats flying above the artificial clutter.