We investigated the interaction between the honeydew-producing whitefly Aleurothrixus aepim and tending ants on shrubs of Croton floribundus (Euphorbiaceae) in a semideciduous forest in southeast Brazil. Whitefly eggs underside leaves were tagged during early March 1998 and randomly divided into two experimental groups: control (ants present, n = 75) and treatment (ants excluded, n = 52). During the 75-d experiment, honeydew-collecting ants visited 79% of the control groups of A. aepim. Adults emerged in significantly greater numbers from control than from treatment groups, the latter being heavily attacked by fungi due to accumulation of honeydew. Complete contamination by fungi was three times more frequent at ant-excluded (39%) than at ant-tended (13%) groups. Control groups with low levels of ant-tending produced significantly fewer adults than those more frequently tended by ants. Encarsia parasitoid wasps were more frequently seen on ant-excluded than on control A. aepim groups. Predatory arthropods, however, were equally frequent in either experimental group. This is the first experimental study to demonstrate ant-derived protection in honeydew-producing whiteflies. Given that aleyrodid honeydew can cause considerable damage to the host plant, our results suggest that the honeydew-gathering activity by tending ants is an important factor mediating such multitrophic interaction.