Four new species of chiggers from Buenos Aires Province and their type hosts are Eutrombicula iellisoni off dove (probably Columbigallina sp.), Euschoengastia azulae off opossum, Euschoengastia trapezoides off fox, and Euschoengastia pazca off Rattus sp. Descriptions and figures are given. The four new species of chiggers described below were collected by Dr. William L. Jellison while a member of the staff of the Pan American Zoonoses Center (World Health Organization), Azul, Province of Buenos Aires, Argentina. Dr. Boris Szyfres of the same organization assisted in some of the collecting. The terminology used in these descriptions has been adapted from Wharton et al. (1951). In extensive studies on trombiculid larvae so much consistency in structure has been demonstrated that use of a stylized terminology and detailed drawings of a few diagnostic features clarify the identity of species. A thorough understanding of this terminology is necessary for anyone doing work in this field. All collections were made in the vicinity of Azul. Holotypes and some paratypes are deposited in the Rocky Mountain Laboratory and other paratypes are distributed to the U. S. National Museum, British Museum, Chicago Natural History Museum, and Institute of Acarology, Ohio State University. Aside from the new species among the 26 lots examined the only other species present was Eutrombicula batatas (Linnaeus) taken from opossum, mouse, partridge, and unidentified bird, 15 January to 21 March 1962. This is first record of the species in Argentina. Eutrombicula jellisoni sp. n. (Fig. 1) Type data: Holotype and five paratypes, RML No. 38096, off juvenile dove (probably Columbigallina sp. ) from nest on ground, 8 March 1962. Other material: Two specimens, RML No. 38143, off Rattus sp., 22 February 1962. Diagnosis: Separated from Eutrombicula batatas (Linnaeus) by one mastitibiala III, two mastitarsalae III, and six setae in first posthumeral row. Body: Ovate. Length and width of holotype, slightly engorged, 237 by 170 u. Eyes conspicuous, of moderate size, the anterior larger, 2/2, in a plate. Anus between second and third rows of ventral setae. Gnathosoma: Conspicuously punctate, the puncta rather sparsely distributed except for a dense area proximally on the cheliceral bases. Blade with tricuspid cap. Palpal setae B/B/NNB; tarsus with seven branched setae, a subterminala, and a tarsala; tibial claw bifurcate, the accessory prong inner and ventral. Galeal seta nude. Scutum: As figured. Puncta large and moderately sparse. Sensilla branched on apical twothirds; PL > AL > AM. Measurements of holotype: AW 70, PW 82, SB 34, ASB 27, PSB 25, AP 29, AM 29, AL 37, PL 45, S 51. Legs: Punctate, puncta large and sparsely distributed. Specialized setae as figured, one mastitibiala III and two mastitarsalae III. Branched setae mostly pectinate, none plumose. Body setae: Dorsal setae similar to scutals, with semiappressed branches, 33 to 46 ,u, the shorter ones posterior, arranged 2-6-6-4-2. Ventral setae 2-2 (sternals) 6-2-2. Named for one of the collectors, Dr. W. L. Jellison. Euschoengastia azulae sp. n. (Fig. 2) Type data: Holotype and eight paratypes, RML No. 38148, off opossum (no further identification), Received for publication 29 May 1964.