During two years (10/92-10/93 and 10/95-10/96) the nesting biology of Isodontia costipennis (Spinola) was studied using trap nests in four distinct areas in the Campus of the Universidade Federal de Minas Gerais (UFMG), Belo Horizonte, M.G., Brazil. Females built linear series of one to six brood cells, weakly separated by wads of grass stems and filled with plant material (grass stems and fibers). Cells were provisioned mostly with nymphs of Tettigoniidae and Gryllidae. Wasp's eggs were laid on the cephalothoracic junction of the prey. The number of prey per nest varied from 1 to 18. Sixty-one adult wasps emerged from the 41 occupied trap nests (69% females, 31% males) and the sequence of sexes in the cells was variable. Thorax widths of females were significantly larger than males (respectively 2.63 plus or minus 0.22 mm and 2.37 plus or minus 0.20 mm). An ichneumonid wasp (Messatoporus sp.) was the only natural enemy found. Nutrition; Diet; Prey; Reproduction; Reproductive behaviour; Ecology; Population dynamics; Population structure; Predators; Animal constructions; Land and freshwater zones; Neotropical region; South America Isodontia costipennis (Sphecidae); Insect prey; Ensifera (Orthoptera); Reproduction; Nest building; Nest provisioning; Population sex ratio; Nests; Brazil; Minas Gerais; Belo Horizonte; Nesting biology Ensifera (Saltatoria); Insect predators; Isodontia costipennis (Hymenoptera); Predator nest provisioning; Brazil; Minas Gerais; Belo Horizonte; Hymenopteran predator nest provisioning no species none