Mygalomorph spiders are rarely found in caves and most of the records appear as accidental or restricted to small populations. The present study took place in iron formations in Carajás region, southeastern Pará state, eastern Brazilian Amazon rainforest. Each cave was sampled twice: dry season (from May to October) and wet season (from November to April). Of the 242 caves sampled, we found mygalomorphs in 98 (40%). The survey yielded 254 specimens, 223 (87.8%) juveniles. Of the 14 species recorded, nine were represented by adults: Dolichothele tucuruiense (Guadanucci, 2007), Hapalopus aymara Perdomo, Panzera & Pérez-Miles, 2009, Acanthoscurria geniculata (CL Koch, 1841), Theraphosa blondi (Latreille, 1804), Nhandu coloratovillosus (Schmidt, 1998), Fufius minusculus Ortega, Nagahama, Motta & Bertani, 2013, and three new are species described here: Guyruita metallophila n. sp., Hapalopus serrapelada n. sp., and Idiops carajas n. sp. Five other “morphospecies” were represented by juveniles only: Ummidia sp. (Ctenizidae), Paratropis sp. (Paratropididae), Bolostromus sp. (Cyrtaucheniidae), Diplura sp. (Dipluridae), and Idiophtalma sp. (Barychelidae). The high number of juveniles suggests two alternatives: some species could be using the hypogean environment as reproductive shelter; the hypogean environment is used as a refuge by immatures. Two species appear to be troglophiles: G. metallophila and H. aymara.