ABSTRACT. Abrão F.O., Duarte E.R., Nigri A.C.A., Silva M.L.F., Ribeiro I.C.O., Silva K.L., Rosa C.A. & Rodriguez N.M. [Physicochemical and microbiological characterization and fungal population in the rumen contents of higid beef steers or wih ruminal acidosis.] Caracterização físico-química e microbiológica e população de fungos no conteúdo ruminal de novilhos de corte hígidos ou com acidose ruminal. Revista Brasileira de Medicina Veterinária, 37(1):7-14, 2015. Instituto de Ciências Agrárias, Universidade Federal de Minas Gerais, Av. Universitária, 1000, Universitário, Montes Claros, MG 39404-006, Brasil. E-mail: firstname.lastname@example.org In cattle fed with diets without vegetable fibers the development of fungi and other microorganisms in the rumen environment can be altered. The objective of this study was to evaluate physicochemical, microbiological characteristics and fungal population in the rumen contents of steers raised on lignified pasture or in feedlot without forage. The experimental design was completely randomized, 50 steers were randomly bred in mature pasture and 20 steers fed without forage. Immediately after collection of ruminal fluid, its physicochemical characteristics were evaluated. Micromorphological examination, direct examination for the detection of strict anaerobes fungi and culture, quantification, identification of aerobic fungi were performed. On direct examination, anaerobic fungi of the rumen, monocentric and polycentric were detected in similar proportions (P> 0.05) for rumen fluid of steers raised in tropical pastures during the dry season. However, structures of these microorganisms were not identified for any of the samples obtained from steers fed without vegetal fiber. The average colony-forming units of aerobic fungi by mℓ of ruminal fluid from confined calves was significantly higher than that of animals raised on pasture (P <0.05). After microculture, the genus Aspergillus was the most frequently identified among the isolates obtained for both bovine groups. The ruminal environment of feedlot steers without forage is compatible with characteristics of rumen acidosis, showing absence of anaerobic ruminal fungi and yeasts and predominance of Streptococcus spp. and mycelian fungi.