This paper describes the morphological and behavioural adaptations responsible for ant-like appearance in eight species (genera Zuniga, Synemosyna, Sphecotypus, and Myrmecium) of salticid and clubionid spiders studied in Amazonian and SE Brazil. All ant-mimicking spiders have body and legs thin, and the shiny integument typical of their models. Light horizontal hair bands and constrictions of the cephalothorax and abdomen simulate, respectively, the head-thorax joint and segmented gaster of ants. The petiole and postpetiole of the ants are usually mimicked by a lengthened pedicel, together with a narrowing of the posterior cephalothorax and/or anterior abdomen. The prominent pedipalps of the spiders often simulate ant mandibles, but they may also be strikingly similar to an ant's head. All ant-mimicking spiders walked in a zig-zag ant-like pattern, and frequently raised and moved about the first pair of legs as 'antennae'. The mimics were found in the same microhabitats (foliage or ground) as their models, and displayed strong avoidance reactions toward the latter both in the field and in captivity. The inoffensive characteristics of the mimics and the noxious traits of their models (strong mandibles, potent sting, hard integument, venomous secretions) strongly suggest that the spiders are Batesian ant-mimics. The detailed structural and behavioural adaptations enhancing ant-mimicry provide strong circumstantial evidence that the selective agents involved must have good visual acuity, and are probably small insectivorous vertebrates (e.g. birds, lizards and toads) or arthropods (e.g. wasps and spiders) which avoid ants. © 1988.331115